I want to restart preparing for IELTS because the last result has expired.
My writing is the weakest in the four subjects. Specifically for the writing, should I start by memorizing good sentences and structure?
I would recommend you to read as more as possible. As they say good writers are good readers. And memorizing structure would help too. Going through sample essays would give you idea how to start
I suggest you learn a couple of complex sentence patterns and then try to incorporate them into your writing.
If you go to the link below and look at conditionals and relative pronouns, that will give you an excellent starting point.