Mainly the felt paper is a precautionary measure until you get your shingles on. 3"-4" overlap is fine. Just staple along both edges or use the tab nails to hold it good along the edge until you get the shingles on. Also when you start your shingles...quite a few just take the starter row and turn it upside down. The way I do it is to cut off the three tabs and put the glue strip at the bottom for the starter. Then when the next shingle goes on top the three tabs are glued down. Have fun!!!! I hate roofing with a passion and seems like every year I end up having to roof something. Oh yea...don't forget the drip edge like Dinger stated.