How can I become an astronaut at ISRO

Astronauts carry out activities related to human space exploration. They work in outer space also besides working at ground stations. As far as I know, only those country provide training to become an astronaut for venturing in outer space activities who have capability to launch manned vehicles in space. Perhaps, presently ISRO has no such capability. However besides NASA, European Space Agency, Japanese Aerospace Exploration Agency, Canadian Space Agency, Russian Federal Space Agency and China National Space Administration provide such training to their nationals.
However the author may target to clear Indian Institute of Space Science and Technology (IISAT) Admission Test (ISAT) for admission in 4 years B.Tech. (Aerospace Engineering) or 4 years B.Tech. (Avionics) or 5 year dual degree (B.Tech. +MS/M.Tech.) program offered by Indian Institute of Space Science and Technology. For appearing in this examination, the candidates have to first clear JEE (Main) and JEE (Advanced).

Indian Space Research Organisation (ISRO) is engaged in space research and related activities. As far as I know, till now ISRO has not engaged in training of astronauts.

I still remembers, Rakesh Sharma and Ravish Malhotra, two Officers from Indian Air Force, were taken to the USSR in eighties and they were given extensive training for one year. Thereafter, Rakesh Sharma was sent to space in a USSR space shuttle. He became the first Indian astronaut.

So, the qualification for becoming an astronaut is not formally defined. However, physical and mental alertness along with high level of technical capability is definitely required for this purpose.