I started studying for the GMAT around December and took it in March. My goal was to knock the GMAT out before starting applications so I didn't have to juggle all the different application requirements with my full-time job.
The very first step I took was buy the GMAT Official Guide and take the diagnostic test. Although this isn't a graded exam, it was very helpful for me to understand what types of questions would be on my exam and a starting point for which concepts I needed to brush up on from high school (e.g., prime numbers, geometry, grammar rules). After that, I downloaded the practice questions online. My first piece of advice is to complete all the practice questions using the online platform GMAC uses for the exam to get acquainted with the software. You'll go through all the questions fairly quickly, and trust me, it's just a warm up.
Target Test Prep is where you are going to get into the meat of GMAT concepts and actually see an improvement in your score. Before this, I would recommend taking the first official GMAT test (can access for free with the Official GMAT books I believe) so you have a baseline score. I scored a 610 on my first try - it was a bit discouraging given my goal was 710+, which meant I needed a 100-point improvement. However, it didn't take me long to realize I would have a huge score jump from familiarizing myself with the concepts. TTP is great because you can take a diagnostic test and then have modules tailored to your weakest sections. One thing I had to force myself to do throughout my GMAT journey was to also continue practicing my strongest sections- these questions would be the ones I strove for 100% accuracy and gave me a strong basis for a good score.
So, as I mentioned earlier, TTP has a lot of modules for every core concept in math and verbal. It is overwhelming at first, but I approached it first by studying the section I was most confident in - math. After drilling down on each math concept (I did most of the modules to make sure I hit all the bases), I would practice a mix of medium and hard questions at the end of each module to ensure I had a grasp on the concept and was confident enough to move on. TTP allows you to even mix up questions among different modules so you can essentially make your own practice test of concepts you want to drill down on more. For the essay, TTP gives you the exact template you need to get a 6.0 on exam day. I scored a 6 on the essay both times I took the exam using their format.
After 4 months of studying, I got a 660 on my first attempt - this was much lower than the practice test scores I was getting (720-730) and I blame it on test anxiety. I hadn't experienced the testing center or took a practice test in a simulated environment. This was a turning point for me in my study patterns. I decided to schedule a test 3 weeks out (advice: minimize time between retakes so you don't forget concepts/strategies) and focus my improvement in reading comprehension and critical thinking, while maintaining math (in my experience, you eventually hit a ceiling for your math score) by continuing practice questions. I didn't focus on verbal as much my first time around because I thought there wasn't really a way to study for it - you were either naturally good at it or you shoot your shot. But this is FALSE. I used TTP's verbal modules to find very effective strategies for the different question types. Once I started doing the practice questions at the end of the modules using the strategies they taught me, I was pleasantly surprised to see they really worked. And to prove it, I saw a jump in my score from 660 to 760 by my next exam, largely due to verbal. This score was higher than any of my practice test scores! I owe it to the confidence TTP gave me.