The key question is - what is your current score with the official PST?
Do one full case from the mck website in 60 min. Check your score. The target score is 22 out of 26 correct. If you do 17-18 correct you have great chances to improve it quite fast. If lower - it will take a couple of months
If your score is high:
- Buy Victor Cheng test prep program - the best materials I've seen so far. It works also for express prep.
- Practice your math. Learn how to multiply double-digit numbers (https://www.youtube.com/watch?v=6ndkkPZYJHo). Learn the division table up to 1/11 (i.e. 5/6 = 83.3). Learn how to work with zeros (E.g.: 4000000 = 4*10ˆ6)
- Do the 2nd test and check the score
If your score is low you need fundamental prep:
- Understand where you have problems (Math, speed, critical reasoning)
- Work on them:
- PST-like tests available online, GMAT IR part with the proper time tracking - for speed and math issues
- GMAT CR and IR parts - for critical reasoning issues
- Speed reading if English is not your native language and you need to improve the speed