Here are few preparation tips you can follow for the same.
Begin Well with knowing the IIT JAM Syllabus & Paper Pattern.
First of all, you need to know the IIT JAM Syllabus & Pattern. As you all know the syllabus is huge so start preparing few months before the exam. Don’t get messed up with the topics out of syllabus. Stick strictly to your subject’s syllabus.
You should make a list of subjects and prioritize them based on the marks allotted to them. Now, create a timetable with the distribution of subjects. It will help you to study the different subjects according to the marks. If the distribution is not prepared, then you usually end up learning random topics, which may lead to confusion at the end.
Try to solve the papers as much you can.

Good Luck!