Before starting to solve questions you should know all basic rules and strategies that you can find in
MGMAT books.
Then you can jump into solving questions. The most important thing to improve your GMAT skills is spending a lot of time in reviewing right and wrong answers to understand the concepts you will need to master in order to be successful.
Personally, I went through all guides, did the examples and now I'm just focusing on solving and reviewing over and over again.
The OG 13 offers around 900 questions, solve them so many times until you know every single one of them. You have enough time until february and by that time you will see a lot of questions popping up in the official test that you will be able to solve because you already know the concept you need from a similar question you know from the OG 13.