After the results of JEE Main And JEE Advanced are out, you will be required to register with the national admission conducting body called JoSAA.

You will be asked to register with your login details (same as that of JEE Main portal). There will be a list of Institutes (only NITs if you could not qualify JEE Advanced and both NITs + IITs if you could qualify JEE Advanced).

You will have to select the institutes as per your choice/ preference.

Then the seats in the different institutes will be allotted to the students on the basis of their ranks.

The same process happens usually in the month of June and July.