What is needed in medical for the JEE Mains counselling?

I participated in last year’s councelling . I think you are talking about the document verification process.
I think following points will clear your doubts-
1) you need to go into the nearest old iit or nit for document verification. I went to iit kanpur.
2) you need to carry all of your certificates i.e your class 12th admit card , marksheet,transfer certificate,migration certificate, caste certificate if you belong to OBC-NCL or SC and ST, PWD certificate if you belong to this category, confirmation paper for seat acceptance fee payment and medical certificate.
3) whole process is quite simple. You just need to go to verification centre and show all the documents and one more important process is to opt for freeze,float or other options for your alloted seat. Keep in mind that if you are getting a good seat of your interest then definitely choose freeze option(in my point of view).
Now let’s move to medical certificate . I was so afraid initially . But you just need to download the form in prescribed format from JOSAA website and then go to a really good government doctor who is registered. You will easily find such a doctor in your locality. Then he will carry out rest of the procedure. As mine doctor said that their main work is to find out if the student has color blindness or not. Believe me everything is simple and you will get a huge satisfaction after completion of document verification process.
Good luck for better future.