Dear student,

If you have completed your aerospace engineering than for joining ISRO or any other psu, you have to secure a good score in GATE Entrance Exam. GATE is a very highly competitive exam which tests your technical skills and in depth knowledge in your area. If you have a good GATE score then you can apply for ISRO from their website.

ISRO also take their own tests and you can apply it from their official site. You can apply for isro by the last of September as the registrations date are extended of recruitment for the post of controller in human space flight centre ( HSFC) at Bangalore.
Also the RECRUITMENT OF TECHNICIAN-B / DRAUGHTSMAN-B / TECHNICAL ASSISTANT is open. You can apply it online.

And remember that aerospace engineers are very demanding and in isro you will get a very good job.

But also note that there are not much vacancies. They take many less students from aerospace engineering. And their vacancies doesn't come regularly.