Preparing for ISRO requires you to firstly cover all previous year question papers and then list all the concepts subject wise. Then prepare individual subjects focussing more on the concepts asked in ISRO and then try to solve mock tests. One can also refer ESE Prelims previous year question papers subject wise for practice. Time is very crucial in ISRO exam so practicing questions in exam environment is very important. So don't just rely on clearing theoritical concepts. Just cover major concepts from each subject fast and then try to solve as many questions as possible untill you feel good about that subject.