If you have got 20 days for the preparation of Psychology only, then you can do it very comfortably up to perfection by dedicating 10-12 hours a day. Fieldsman's book is very easy. Go through the study plan which you will find here and then read the topics according to that. First read the topic thoroughly in the Fieldsmen book and then prepare your own short notes but if u r not quick enough then highlight important points in the book so that it might be easy for u to revise it on the day of exam or before the day of exam. Best of Luck!