To install a skylight, measure the skylight, create a frame between the rafters, and drill screws through the ceiling at each corner. Remove the shingles, and mark the lines between the screws with a chalk line. Cut along the chalk line with a circular saw, install sheathing around the perimeter of the opening and install self-stick membrane on the bottom edge of the opening. Nail the skylight into place with galvanized nails, and seal the skylight.
To create a frame, work inside the home, and double the rafters on each side of the skylight. Install a doubled header to the top of the frame and a sill to the bottom of the frame. While inside the home, drill long decking screws through each corner of the skylight to mark the cut-out on the roof.
Remove the shingles located between the screws and from the surrounding 7 inches of roof. After nailing the skylight in place, install self-sealing membrane around all sides of the skylight. The membrane should extend 6 inches past the edges of the skylight.
When installing the shingles, layer the top half of the shingles with L-shaped metal flashing around the perimeter of the skylight, and install counterflashing. Install saddle flashing around the skylight by sliding the edge of the flashing under the underlayment, nail the saddle flashing in place and install shingles over all but 4 inches of the saddle flashing.