What is the procedure to become Astronaut after B.Tech Mechanical Engineering???

Since you have completed B.Tech Mechanical Engineering you are qualified to become an astronaut. Till from mechanical engineering also you may become astronaut but for this you have to do Master degree(M.Tech) is aerospace related subject. After completion of your M.Tech with some proper training you may become astronaut.

To become an astronaut you should fulfill the following eligibility requirement:

Educational qualification:
-You should hold a bachelor's degree from an accredited institution in engineering, biological science, physical science (like metaphysics, astrophysics, etc.)or mathematics with excellent academic record.
-An advanced degree preferably a doctoral degree is highly desirable.

Physical standard:
Height - should be between 62 t0 75 inches
Vision - 20/100 or better uncorrected, correctable to 20/20 each eye

If you meet the above eligibility requirement then you can apply for an astronaut's job in NASA or ISRO. After applying you need to pass an entrance exam in order to become an astronaut.