First he took the back off . . .
Then he pulled all the staples out . . .
He cut the padding to fit. We used quilt batting, but you could use foam. Mostly just use whatever makes your butt happy.
Then cut the fabric larger than the piece you are covering. Check the size of the piece you took off. Pull it tight and staple on backs side. Watch how you pull, not to cause any wrinkles.
The final product.