Measure the width and length of the roof and multiply them together to get the total square footage of the roof. This will determine how much underlayment and roll roofing will be necessary for installation.
Roll out the felt underlayment beginning in either corner of the roof. Insert a nail every six inches around the perimeter of the roof to secure the underlayment. Add a nail every 18 inches along the inside of the felt to secure it to the roof. Continue spreading out the underlayment until it has completely covered the roof and make sure there are no nails sticking up.
Cut two 17-inch strips with a utility knife out of the roll roofing measuring the length of the eaves of the roof. Apply a layer of roofing cement to the eave of the roof and spread it out with a paint roller. Lay the strip down on top and secure it to the roof by running a roofer's roller over top of it. Insert two rows of roofing nails every 12 inches along the strip and repeat the process for the eave on the other side.
Lay the roofing down and spread it out so it is covering the entire roof. Nail the roofing down to the first starter course by inserting two rows of nails spread 12 inches apart 4 1/2 inches and 13 inches from the top of the starter course.
Roll back the roofing and spread roofing cement over the entire area. Spread it around with the paint roller and an extension pole and set the roofing back over top of it to adhere it to the roof. Insert nails throughout the roof every 4 inches to secure it to the roof and nail the roofing to the starter strip on the other side just like the first one was secured.