Knowing the format of the PSAT 8/9 test will help. The test has four sections: Reading, Writing and Language, Math (no calculator), and Math (calculator). There are no negative points for getting an answer wrong. Guessing the same letter or bubble for every question will get you a better score than guessing an answer for every question you don't know. Now if you can just identify 10 questions in Reading, Writing and Language and Math that you can answer correctly, you will raise your score to 700 assuming you picked the same bubble for the rest of the question. So, in short you can finish a 2 hour 30 minute test in less than 30 minutes to score 700.
Clear everything in your agenda for 2 hours and 30 minutes to sit down and take a full length practice test like you would on test day. Once you do this and score the test, you will know your baseline score.
Identify your weakness areas by going through the questions you got incorrect. Calculate the number of questions you got wrong in each section of the test. Now if you can reduce your errors in each section by half, you will be able to raise PSAT score by 200 to 300 points minimum. Knowing this set your target score.
If you can spend between 10 and 20 hours over the next month, you can improve your score by the 300 points you identified earlier. Start with the Reading session and spend 1 hour a day for 5 days to review and practice questions. Week two move to the Writing and Language section and repeat the same 1 hour of practice for 5 days. Week three work on the Math section for 1 hour for 5 days.
Pick another practice test and complete it in 2 hours and 30 minutes. This time, you can employ the same strategy as before but look for the questions you identified as weakness and spend a few extra seconds to answer them. If you can get 5 to 10 additional questions correct in each section compared to your previous attempt, you will be able to reach your target score.