NDA consists of written exam and if you clear NDA exam then you are eligible to sit in 5 days SSB 

Interview.

Once you clear SSB interview and get recommended you will have one medical test.

After clearing medical test then you will be in NDA.

For more info read the following articles-

http://ssbcrackexams.com/how-to-pass-nda-written-exam/

All the best!!! 

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