How should I practice for the JEE Main to get 300 or above?

Your preparation should be top-notch if you are aiming for more than 300. First, go through the NCERT books thoroughly. It will be better if you practice solving JEE Advance problems. JEE Main Physics, Mathematics, and Chemistry are relatively straightforward compared to JEE Advanced. So, you will be well-prepared. Solve difficult problems, and make note of the different concepts you learn. Give regular mocks and time-bound tests. 

Scoring 300 or above is a big thing considering the level of competition of the JEE exam in this country.

• The first thing you need to do is make up your mind and dedicate all your attention  to the preparation of the exam. 
• If you are a coaching student, and are running parallel to the syllabus, then focus a lot on your revision.
• Prepare a study schedule and follow it religiously. 
• To achieve a score of 300 you need to target a 360. Prepare keeping that number in mind.
• Practice is key. There is no substitute for practice, the more you practice the more you become familiar with every concept. 
• Do not skip topics or chapters as that could hamper your target score. At least try to study the important parts.
• Take mock tests and complete it within the given time limit. This will help you during the final exam.

Make sure you are not wasting your time involved in any distracting activities like games, social media, etc. because they take up a lot of your time and energy.