You may get in through the following entries-

Registration of Students with Scholastic Aptitude (ROSSA) entry if you have passed your post graduation in first division.

Orientation course for engineering graduates and science post graduates (OCES) at BARC Training School and its affiliates.

KS Krishnan Research AssociatesAssociates

Direct recruitment through open advertisements.

For more information, you may refer to this-

https://www.google.com/amp/s/engineering.careers360.com/articles/isro-drdo-barc-dae-how-work-here/amp