I generally recommend only that you edit yourself after a very long break – and stick to broad concepts.
Unless you have already internalized the grammar rules – or are willing to look them up when you’re not absolutely sure (professional editors do this all the time) – it is next to IMPOSSIBLE to copy edit or proofread something you have written, because it will look just fine to you. I’m not just saying that because as an editor, I like to have work.
I am an author as well. Missing words and misplaced commas are the worst for self-editors. Your brain puts missing words in the right place. Some people also only understand commas as breath pauses and, well, you’re reading it to yourself in your own voice, so where you pause, well duh, a comma should go there, right?
Not necessarily. However, if you do want to edit yourself, do so ONLY after a significant break from having written it. I recommend a week for every 10,000 words you wrote. So a novel is going to get set aside for a month, probably two.
DON’T EDIT FROM THE BEGINNING
When you do decide to edit, start with the END. This avoids a lot of the mental “stick in the missing word” or assume that the idea is there.
Isolate each sentence and ask yourself if it follows all the rules of a sentence. If you don’t know what those are, get Strunk & White’s Elements of Style, or another grammar guide like The Chicago Manual of Style. Finally, invest in a dictionary.
Saying “it looks right” won’t resolve it’s/its or their/there/they’re confusions or plural/singular construction if you don’t recognize when you’re looking at a compound subject or a subordinate clause. You also will miss past/present verb tense slip ups as well as point of view errors.
Do not rely on Microsoft Word, or ProWritingAid, or Grammarly. They are all programmed to apply a binary (right/wrong) interpretation of grammar rules. Fiction, however, does allow some flexibility in construction for things like emphasis and drama.
As you go sentence by sentence, If the sentence has ideas that don’t make sense, ask yourself what information is needed, then expand your view to look the sentence before and after and see if these provided the information for that initial sentence to make sense. If they don’t, add in the necessary ideas. You may end up reworking a sentence or a set of sentences so that one clause ending a sentence instead is moved to the beginning of the next sentence. Or the start of the sentences ends up moved to the end of the previous one. Such movement often changes the verb forms, becuase you’re turning the subject of a sentence into the object of another one.
Doing a self-edit by starting at the end will also help with pronoun/antecedent agreement, as well as clearing up who is saying/do which words and actions. It will also allow you to determine if a fragment is just fine, despite the grammar rule (fragments can creating emphasis and drama) or if the fragment needs to be filled out to complete.
This isolated sentence/paragraph approach will also allow you to see if subjunctive clauses or coordinating phrases are doing their work in the right place in the sentence or if they need to be moved before or after another part of the sentence in order to create a clear communication to the reader.
And please, your readers will thank you if you hire an editor because you don’t/can’t do a self-edit. Your writing deserves to be read AND understood.