Hi

Every year JoSAA releases the format for various documents among which the format for the MEdical Certificate is also included. You have to download the Business Rules file from JoSAA 2019 official website and search for a certain annexure which contains the entire required format for the Medical Certificate. Take a printout and fill all the details correctly and carefully and get it signed by the doctor as well. 

Click this link to know in detail about JoSAA Counselling 2019: https://engineering.careers360.com/articles/josaa-counselling.

Best of Luck.

Hope this helps.

Cheers!