Once you're familiar with the format of the test, take one of the practice tests from GMATprep - download the software at mba.com
This will give you a good base score and understanding of which section you need to work more on and which sections you may not need to work on as much.
Then review the questions you missed - this will give you some idea of what core concepts you need to review.
Then go through the books, and take a practice exam every now and then to see improvements (note that different CATs will give different scores, so take the actual scores with a grain of salt, except for the GMATprep scores; GMATprep is extremely accurate/predictive of your actual performance).
remember to categorize/record every single practice question you do in any of the work books, figure out what you're missing, learn the concepts behind these questions, and keep going.
When your practice test scores are in the range of your ideal score - take the test.
If you do it right, then this should take around 2-3 months (to go from 6xx to 700ish).
If you're already scoring in the low 7xx on the initial GMATprep exam (meaning you've got a pretty good foundation/fundamental understanding of the concepts), focus your study on your weak area (DS/PS/SC/RC/CR) and the drill those and the concepts behind the questions.
You'll need probably 2 weeks to go from the a low 7xx score to a mid/high 7xx score if you can address your 1~2 weak areas.