Dear Aspirant, for admission in NITs after JEE Mains-

The process starts after announcement of result of JEE Advanced of that year.

JOSAA is the official committee responsible for smooth process for students to admission to IITs, NITs, IITs and  other GFTIs.

JOSAA official website :

https://josaa.nic.in/WebInfo/Page/Page?PageId=1&LangId=P

Here they'll  announce everything about  the process.

You can also check about opening and closing ranks(2020) of all the NITs here :

https://josaa.nic.in/Result/Result/currentorcr.aspx

This will help you get an Idea to make a preference list among all options.

Generally , one needs to do the registration on JOSAA then there are some mock rounds which give an idea. After that there are 7 rounds in each round a college and branch is allotted according to you rank and preference list. Keep your all documents ready before this counselling by JOSAA starts , in one of the rounds there will be a document verification process. Meanwhile you'll get idea about the college you'll get into so keep an eye on their official website also, it'll give you further process idea.

Check more here :

https://engineering.careers360.com/articles/jee-main-counselling

Also you could use this tools:

JEE MAINS COLLEGE PREDICTOR-

Hope this helps!

ALL THE BEST!