The most important thing writing a league is to establish clear and vibrant identities. You will always have a very good idea of who your clubs and players are in your head, but realistically, none of your readership will share in that knowledge. Every club has to have a reason to exist, both IC and OOC, and you should go to every effort possible to establish their brand.
You probably already have some idea of at least a few clubs, if you've focused on your national team first and have described some of your players as coming from those clubs. But you'll still need to build on those names, and are likely to need to bulk out your league with new creations, to say nothing of lower leagues.
These are the core factors to take into account when you are constructing a club. These are presented in a particular order, but your actual thought process may well shuffle them around.
NAME / LOCATION
This is probably the part you thought of first, and already had down on paper. Most club names consist of a location and, usually, a title. Think of real-life examples - Manchester (loc) United (title); Real (title) Madrid (loc); Sydney (loc) FC (title); Kerala (loc) Blasters (title); Bayern (loc - region) Munich (loc - city); Liverpool (loc; no title).
Try to have some variety, both in terms of location and name. Obviously there's IC caveats here - maybe your country has one huge capital city and everything else is an afterthought. But even then, don't just have Capital City, Capital FC, Capital United, AFC Capital and South Capital. A few of those are fine, but you can still name others after regions of the city, or other things.
A name can tell you a lot about a club. To go back to RL examples, think about Real Madrid (aka 'royal Madrid', big and prestigious) vs. Atletico Madrid (aka 'athletic Madrid', which conjures up a more physical, workmanlike image) vs. Rayo Vallecano (a much smaller Madrid side, named for the neighbourhood of Vallecas). Similarly, looking to the MLS - from the name alone, compare the loud, brash Los Angeles Galaxy (as in 'of stars') who revel in their American identity and go out to buy stars to the more low-key, grassroots, Europhile Los Angeles FC, with their more 'traditional' European title.
We'll be keeping on with our example of the Ceynes Super League, and looking at three clubs in particular. Imperial Falston sound big and impressive, and name-drop Falston, one of the two relevant cities in the Isles. They're probably going to be big, flashy heavy-hitters, especially as they've provided a few players for the Ceyne Isles national team. Roschester Pride represent the smaller of the two relevant sides - they have a less 'prestigious' title, but a loud, flashy one. And not every side is going to be good - it is clear just from the name that AFC Mosstown are a pack of bland bottom-feeders, struggling to survive.
In many real-life leagues, England being especially guilty of this, 80% of the clubs wear primarily red, white and/or blue. And, okay, these are nice colours. But it gets a bit dull. While club colours in real life are generally just an accident of tradition, you have the luxury as an NSS RPer of constructing a varied palette.
If you're using any kind of graphics, it also helps to use different shades. Instead of all your red sides having that classic 200/0/0 RGB red, give one a slightly more orange tint, and another a darker, more royal shade. Also, you can mix and match using stripes, hoops, chevrons or sashes to switch things up a bit. If you want to, you can use various graphic design programs to make it so that their kits and/or crests can be viewed by others. This is something I would probably not worry about for at least another couple of cycles, and you don't have to do it at all. But visual identity can still be conveyed even just by writing what colours they are.
Imperial Falston wear all-white with purple trim, very regal colours, very Anglatian colours, reflecting that identity.
Roschester Pride wear purple - a more vibrant shade than Imperial's trim.
AFC Mosstown wear apple green, reflecting ennui.
How strong is the club? Strength obviously fluctuates year-by-year, as players arrive and depart, but power tends to stick. Good players would rather join a successful club, which begets more success. There's a correlation with a club's size, as well - it's a lot easier to be good if you have a huge swathe of a capital metropolis to recruit locals from and sixty thousand fans through the gates every week - but money is the main factor. RL is filled with examples of obscure sides in the middle of nowhere - Basiksehir in Turkey, Hoffenheim in Germany, Ludogorets in Bulgaria - bankrolled to relevance despite a tiny fanbase and no real history of success.
This is a lot less true in salary-capped, franchised leagues. When a powerful team wins the league, players will come looking for bigger contracts - and suddenly the same side no longer fits under the cap, and some are squeezed out. I don't actually recommend simulating a salary cap or keeping track of contracts unless you really like numbers, but it's just an example. Even then, though, there will still be sides that are more consistently successful, and certainly more famous.
In any case, you probably have some idea of what the big clubs are, and what the strong clubs are - which isn't a 1:1 deal, but most big clubs will be strong. Also, don't feel shackled to the whole concept of it being your first year in UICA. Your nation didn't start playing football the moment when they joined the Baptism of Fire, did they? And footballers need clubs. It can be useful in your RPs to talk about 'modern era' records, aka your first UICA-submitted league, but you can still make reference to sides being far, far older.
Imperial Falston are the powerhouses of the CYI League. It's a young league (in IC terms), so they aren't old as such, but they have a huge fanbase, a lot of money and an expectation of success.
Roschester Pride are probably not a title challenger, but they're still a respectably strong team that prides itself on bringing through youth. They're something of a selling club, though - Imperial often buys the best players that Pride bring through, securing Imperial's dominance and Pride's finances. They're a midtable club, but a particularly good batch of them could aim for the UICA places, and they're probably safe from relegation barring a freak result.
AFC Mosstown, meanwhile, are a small, derelict club from the middle of nowhere. Success, for them, will be escaping relegation.
Including, but not only, the style mod. How does your team play? What is the game-plan? What formation does their strongest XI come out in against a standard opponent? These are of course subject to change year by year, especially with a change of managers. But it's good to keep a common thread between seasons and managers, and try and keep a core identity intact.
You don't have to publicise the style modifier, and it's a bit of an immersion-breaker. Nobody in real life looks at clubs and rates them on a scale of -5 to +5. My personal advice - though this is very subjective - is that, at least with NSFS style mods, it's best to stick to a range of -2 to +3. I haven't crunched the numbers, but I feel like an average of +1 will get you around a 2.8 goal per game average.
Imperial Falston are influenced by the attack-minded Anglatian style of play, and combined with their financial might, they aim to dominate the rest of the league both on the scoreboard and in terms of possession stats. Unlike some possession-heavy styles, they are actually trying to get the ball moving forward, though. They spend most of their money on forwards and creative midfielders. They will use a +2 modifier, and play in a 4-3-3 formation.
Roschester Pride are fun to watch as well, though they don't have Imperial's financial clout. They rely more on pace and power going forward, favouring a young and impetuous side, pressing high up the pitch and sometimes neglecting defensive signings. They will also use a +2 modifier, despite their different style of play to Imperial, but their more fast-paced, physical game lends itself more to a 4-4-2. This also contrasts against their more dour, defensive rivals, City.
AFC Mosstown do not have the budget to buy standout creative players, and if they somehow had one, they'd probably sell them to bulk up the spine of the team. They respect hard work and discipline, and consider any away point a good result, no matter how ugly it is. They get their goals by playing direct, route-one football, and have a good understanding of set pieces. Mosstown will play a dour 4-5-1 formation, and have a -1 modifier.
An easy way to add spice to your league is to add rivalries. Most of these in real life stem from proximity, 'derby' matches between clubs in the same city. But there's plenty of reasons for two clubs to hate each other. Often if there are two traditional giants of a league, their match will be something of a 'classic', and there may also be other political factors driving clubs against one another. Matches between rivals are instantly more important, generally more violent, and they'll only rarely (if ever) transfer players between each other. Expect someone who does make the switch to be called a snake or a Judas by their old fans, and perhaps not even accepted by the new - all great RP fodder.
For a real life example, take Barcelona against Real Madrid. This is a rivalry so fundamental it seems like it must have been created by an RPer allergic to subtlety - Real wear all-white, Barcelona wear a cacophony of primary colours. Real are from the capital, Barcelona from a fiercely independent state. Real's identity is based around buying stars and winning at all costs, Barcelona's around a holistic, youth-based approach and winning with style. Real were the fascist General Franco's favoured club; Franco was known for repressing the Catalan region that Barcelona is a part of. They are chalk and cheese that just so happen to be the two most powerful clubs in Spain, and probably the world. No wonder they hate each other!
Imperial Falston is pretty much just disliked by everyone outside of their fanbase, as money-hungry giants unwilling to break their stranglehold on the league. Anyone in Ceynes tends to support their local club first and whoever's playing against Imperial second.
Roschester has two major clubs, Pride and City - who are a more traditional, conservative outfit than the energetic, grassroots-based Pride. The Roschester derby is known for flamboyant displays of banners and pyro, and only the occasional street riot.
Nobody really cares about AFC Mosstown enough to hate them.
The main guiding principle in all of this is contrast. Trying to get people to remember every club you have is... hard. Can you honestly name every club in every league you semi-follow? So all you can do is make it as easy for your readers as possible, and that means making memorable, distinct identities based around core things. Just to get your foot in the door. I've put all these reasons above for why X is Y, but with our examples, what will really stick around is:
Imperial Falston: Big, rich, marquee focus, hated by everyone, strong Anglatian identity.
Roschester Pride: Decent, youth-based, attacking football, flamboyant fans, hates City.
AFC Mosstown: Small-town, struggling, poor, based around hard work and physicality.
Everything else is built on these foundations. That shouldn't discourage you writing long histories on what each club means, if that's what you're into - these are genuinely a joy to read. But that's a lot of information to absorb, and there's... a lot of leagues for people to keep track of. Find your core principles and reinforce them constantly.
But who plays for these clubs? And how Next time, we look at ROSTERS.