Every Year ISRO conducts their own Entrance examination for the recruitment process. Qualify the exam and then you will be called for the interview round.

If you have good GATE score then you can join top IIT's where Isro directly hires from the campus

You can also join IIST through GATE and then get recruited by ISRO on the basis of CGPA and your Extra Curricular.

If your rank is under 100 then you can join IISc, directly sponsored by ISRO.

All the best!!