Prepare to Shingle
If you are re-roofing an older house, be certain that your old roof boards are sound and secure. Replace any wood that has rotted. You may even wish to put a whole new layer of plywood or OSB sheathing over the old wood. On a new house or structure, you will already have these secured to the rafters. 7/16 OSB will do. When you put it on, begin at the top and apply it in a brick pattern. This adds to the stability of the structure.
When you have a smooth surface to work on and you have cut vent holes, added vents and pipe flashing, it is time to address major obstructions like valleys and chimneys. Valleys are simply the line where two roofs come together. The valley should be lined with flashing. Flashing is just sheet metal. You may be able to buy it with the crease in the middle. If not you should make this crease yourself by folding over the sheet lengthwise, then unbending it. When handling flashing, be sure to use gloves as the edges are sharp enough to cut through the skin. Lay the flashing in the valley, starting at the bottom. Nail it into place with roofing nails about 1 inch from the outer edge of the flashing.
Now would also be a good time to add the drip edge. This should be located along the entire outer edge of the roof line. Place the piece so that the widest surface rests on the roof. There should be a half-inch to inch overhang, the metal will curl in and then bend down at 90 degrees. This surface should rest firmly on the side of your lowest roof board or sheet of plywood. Nail this piece near the top at intervals of every two or three feet with roofing nails. At the corner, cut into the drip-edge as shown in the illustration and bend back the facing piece. The drip-edge should go around the corners as well as up the rake boards (the face of the gable).
Bring felt paper up to the roof and lay it across the surface horizontally beginning at the bottom. Because felt paper can be slippery and can tear when walked on excesively, put it on row, by row just ahead of the shingles. Like everything else on the roof, there should be a bit of overlap, with the bottom of each piece over the top of the next lower piece. Felt paper adds to the impermeability of the roof, but it is primarilly a way to protect the plywood, OSB, or roof boards from the glue and tar on the shingles. This makes them easier to take up 15 to 20 years from now, when you will be doing this job again.