Basically what nate said. You need to blind review and analyze what makes the right answers right and wrong answers wrong. I personally used a spreadsheet for this and it was pretty helpful.

I wrote it recently and can give a bit of insight.

The writing section essentially presents a scenario and then asks you a critical question about it. You then go on to write an essay and argue for a certain thesis. You'll have 4 sources that LSAC provides as "perspectives" that you can reference in your essay plus any general knowledge that you have.

If you have LawHub Advantage I'd suggest doing one of the prompts but its not something you have to study intensively for like the multiple choice LSAT. As long as you have solid essay writing skills you should be good.

Given that you've identified NA/SA/Flaw as weak points, I think that's a great start. In my opinion you should take a look at your process and see what's going wrong.

For SA questions find the premises and the conclusion then find the gap. The correct answer logically fills that gap (aka smoothly leads from the premises to the conclusion).

For NA questions find the premises and the conclusion then find the gap. The answer is something that the argument can't exist without (try the negation test and see if that works for you).

For flaw questions find the premises and the conclusion then try to work out what mistake the author is making.

For all of these questions having some idea as to a prephrase/prediction can help you to not aimlessly look at random answers and be more structured. Looking at common LSAT flaws might be a good start as a lot of them are recurring (sampling issues, correlation-causation, mixing up necessary and sufficient conditions, etc.).

I'd push back slightly against using GPT because in my experience it does get conditional reasoning wrong and I'd rather go to other sources for more indepth explanations. Wishing you all the best!

-Amaresh