Hey,
For the verification of the required documents,the candidates have to report at their respective reporting centres as per their alloted institutes. There are eighteen reporting centres for IIT seat allotment.At the reporting centre,the candidates will have to go through document verification and pay the admission fee.
Follow the link given below for the detailed information about the list of the Reporting centres of JoSAA:
https://engineering.careers360.com/articles/josaa-reporting-centres/amp
Hope this helps.
Good luck.