How to study
1) DO what works for you. This seems simple, but it's important. There is a lot of advice on the internet and it's important for you to find the method that works for you even if it's not the suggested approach by someone else.
2) In general, studies have shown that studying more frequently for shorter periods of time is better than studying less frequently for longer periods of time. I think about 1-2 hours is the best time length for one GMAT study session. It's long enough that you can get good work done, but not so long that your brain starts to get overloaded and can't retain the material as well.
3)When studying, first read a chapter of a theory book, then do some (and possibly all) of the questions given at the end of each chapter. Take notes, keep track of your work, and possibly make flash cards (if you find flash cards useful). Then do a few OG problems. Every time you have a problem write what the problem was down on a piece of paper ("I forgot this formula, I used this technique incorrectly, I messed up my timing, I didn't know how to make a guess"). Then two weeks later go back over all of the problems you had trouble with. Reviewing the problems you had is perhaps the most important step in the process of improving your GMAT skills.