Make a proper Time Table - The first and foremost is that you make a timetable and stick to it. The thing with making a proper timetable is that you will have an exact idea of what you are required to study and the time required for it.
Seek guidance - Sooner or later you will run into a concept or so which will give you trouble and then it is best you seek guidance from an instructor or a teacher.
Concept clarity rather than rote learning - When it comes to engineering entrance exams, more than anything it is essential that you have a clear idea of the formulas and concepts rather than rote learning of things for the papers.
Notes - It is very important that with each covered topic and chapter you make small notes or a comprehensive list of formulas that will come in handy at the time of revision.
Revision of Class XI and XII concepts - Since most of the engineering exams will be for admission to undergraduate courses and so the syllabi from class XI and XII becomes the testing ground to check students.
Sample Papers - It is important to practice the sample question papers from an institute of repute; quality is important and not quality.
Mock tests -
A Mock Test will be able to showcase the student's abilities as it not only provides a similar feel to real exams but also helps in eliminating the exam.