Hi Abdulrahman,
as general practice for the PST, I would recommend the following approach:
- Try to find at least 5-6 practice cases online. There are several available for free, if needed you can purchase additional ones
- Do one of the tests immediately to check your score. As the passing score is around 70% and you have 26 questions, your target score should be 18+ (keep in mind you are likely to score a bit less in the actual test than your average score at home due to time pressure/nervousness). In the first test you are likely to score lower than that - don't worry, that's pretty normal.
- Identify the type of questions where you made more mistakes. There could be different reasons why you are doing mistakes and there is probably a predominant type of mistake you are doing (eg slow math or weak critical reasoning). You have to identify the reasons for the mistakes and a way to fix them.
- Continue the preparation with the remaining tests. After you have identified the main reason for mistakes, you can continue the preparation with the other tests before the interview - I would allocate them at a regular time distance, with an increase in frequency closer to the day of the test.
- Go again through the questions where you did mistakes at regular intervals. Be sure to keep a "failure" spreadsheet, where you report all the mistakes and classify the reason for them each time.
The key areas where you may do mistakes and may need to practice are the following:
- Time management. Commit to an amount of time for each question, and go on if you surpass that time. If you do not set discipline, you will end dedicating too much time to some questions (these tests are sometimes structured with questions which are supposed to be skipped and reviewed at the end). If time and test allow, you can then go back to the questions at the end.
- Quick math. I would recommend practicing with online tools on a daily base to improve. It is better to allocate a small amount of time daily rather than to practice intensively few days only before the interview
- Quick reading. Get a Harvard Business School case or an equivalent long one, give yourself 2 minutes and check how much information you can absorb. Then repeat until you get a sufficient level of accuracy. You can also check speed reading tactics (eg Tim Ferriss ones) and see if they work for you.
- Graph interpretation. You need to practice on how to derive quickly insides from graphs. Take some random graphs, give yourself 30 seconds and check if you can get the main insides from them. Then repeat until when you get a sufficient level of accuracy
- Critical reasoning. The GMAT critical reasoning section should be a good support as practice
Hope this helps,
Francesco
Crack the McKinsey PST by PrepLounge 
