The thought of cutting a hole in your roof needn't conjure up images of dripping water destroying your living room carpet, as long as you know how to seal that opening against the vagaries of nature. And speaking of nature, do the roof work on a day with zero rain probability, and plan on having the skylight installed by the end of the day. Even then, you'd be wise to have a tarp handy, just in case.
Walk very carefully when you're on the roof - the fewer shakes or shingles you disturb, the better. But put your own safety first. If your roof is moderately sloping, secure a foot plank or ladder.
Cutting through Roofing Materials
Though you can use hand tools to cut through a roof, a circular saw or a reciprocating saw is easier and faster. A combination blade is best for cutting through wood shingles or shakes. You may find that a utility knife works better than a circular saw to cut through asphalt shingles.
Use safety precautions when working with any power tool: make sure the equipment is properly grounded, wear goggles or safety glasses, avoid awkward positions, and keep out of the line of the blade. Be alert while sawing-a blade that binds can throw you off the roof.
To cut through shingles or shakes for a curb mounted skylight or through built-up roofing for a self-flashing skylight, adjust the depth of cut on your saw so the blade cuts through the roofing material, but not through the wood sheathing underneath. Resting the front of the saw's sole plate on the roof, align the saw blade with the chalk or pencil mark or the knife cut.
Turn on the power and lower the saw until the sole is resting on the roof. Saw slowly and steadily along the marked line until you reach a corner. Repeat for the other sides.
After you've cut around the opening, pry the roofing materials loose with a crowbar and hammer; save asphalt shingles to use for patching around the skylight. Peel off the roofing felt to expose the sheathing.