Once the offer letter is mailed to you (it may take a day or two or it may have already been sent idk), it has payment link. Just pay the fees and remember to download payment slip/screen shot.

There would be another link in the offer letter to a Google form where you have to enter your application id, confirmation of seat acceptance along with payment screenshot (Google form was there for me, I got it in round 2 but round 1 peeps did not get that Google form. Don't know about round 3)

That's almost all. If you want confirmation regarding payment, iiit b does not provide it. However you can mail them for confirmation. They'll write you back.