Hey kalyani.

let me suggest the basic method for preparing for all these exams. Go through the syllabi, pattern and prevvious exam question papers of these exams. Understand carefully. Plan and divide the syllabus carefully. Practice and plan time for each section so it will make the exam taking easier.

please follow the links below for detailed guidance on preparation and tips to ace the exam:

for JEE: https://engineering.careers360.com/articles/jee-main-preparation-tips

for CET: https://engineering.careers360.com/articles/how-prepare-for-mht-cet-2019

for NEST:

https://university.careers360.com/exams/nest

hope this helped

all the best