The most important thing to remember.
Basically, you're gonna want to look for 3 types of errors in your work. Spelling errors, punctuation errors and grammatical errors. The mistake a lot of people make when proofreading is that they try to look for all three types at once. Don't do that. Follow this method and break down the task into more manageable chunks.
If you are dyslexic then I would recommend using something like a ruler and holding it under each line of text as you read. Often people with dyslexia will struggle to focus on one line of text at a time. Using a ruler helps with this. There are also colour overlay programs you can get - plugins for Chrome. Search for nOverlay - it's the best I've found.
1) Start by looking for grammatical errors.
Read every sentence clearly and carefully - out loud if you have to. Does it make sense?
Most grammar errors are with verbs (the doing words or state of being words) or plurals. Here is a short youtube video on verb identification if you have trouble with this.
Plurals are when you should, or should not, refer to one or more of something.
Verb/subject agreement is another area where people have problems. They will write things like "We was going out" or "I were busy that evening." Here, the verb (was or were) is the wrong form to use with the subject (we or I). Here is a worksheet that explains this very well if you are struggling.
If you require more assistance with grammar, this website is a very good place to start:
2) Move on to look for punctuation errors.
Full list of resources for learning how to use punctuation marks:
The biggest mistake made by people when it comes to punctuation is usually not punctuating their sentences properly. Put basically, there are three types of sentences. Simple, complex and compound. More on this here: https://www.youtube.com/watch?v=hWmKnrtlTHU
People often create very long sentences with far too much going on in them. As a rule of thumb, if you've got more than 3 clauses (complete thoughts) in a sentence, it's far too long. These are called run-on sentences and there is an excellent resource on them here: https://www.gcflearnfree.org/grammar/runon-sentences/1/
2) Finally - spelling.
There's no easy way to say this, but spelling rules are just kind of something you have to learn. Having said that, there is some help out there.
Homophones - words that sound the same but are spelled differently and often hold different
General spelling assistance: https://howtospell.co.uk/spelling.php
Now, you can often build up your own mnemonics to help with spelling. This is how, for example, I remember the difference between their, there and they're.
There - again I remove the t. I'm left with here. There often refers to place or location, so that's sorted, isn't it?
They're - I work this one out by process of elimination. If it's not the other two, it must be this one.
Fortunately, spelling errors are the easiest errors to correct. Punctuation and grammar mistakes often involve having to re-write or re-do entire sentences. Spelling? That's a matter of just typing in the right letters!
So there you go. Remember - don't look for all the errors at once, go through systematically. Read once for grammar mistakes, again for punctuation and once more for spelling.
You've got a tonne of resources there to get you going with. So no more excuses for badly-proofread stuff!