In order to crack NDA exam you should start with making a systematic plan. Here are some preparation tips for NDA exam:

• Everyday 5 to 6 hours study is suggested on regular basis.
• Distribute the syllabus in chunks and prepare accordingly. it shall be less hectic to you.
• Practice previous year question paper properly.
• Time management is a key, analyse how many questions you are able to answer in minimum amount of time.
• When practicing previous papers think once before guessing on an option, since negative marking is implied here so every wrong answer will cost you extra 1 mark.
• Learn shortcuts, Learn tricks, in mathematics on few questions answer can be brought up by looking at the options i.e. instead of solving the whole questions take the given options and solve the question around those options.
• Read newspaper, magazines, journal etc. This shall help you with grammer and current updates as well.
• Study in groups, it help you in sharing the ideas and grasping the concepts quick.

Hope this helps you to achieve your goal.

Good Luck!