Hello ,

Whole process of counseling is quite lengthy to explain , you can refer to this link and read the process in detail :-

https://www.google.com/amp/s/engineering.careers360.com/articles/josaa-counselling/amp

Now talking about format of certificate you can get whole detail on JEE Advance website https://jeeadv.ac.in for IITs and for NIT you can visit JOSAA website https://josaa.nic.in/webinfocms/Public/View.aspx?page=73 but for income certificate there is no specific format because most states have their own format for providing income certificate but still you can check about it on JOSSA guidelines .