How can you cut a roof in a shed for a skylight?

If you are talking about a small tool shed; then measure distance between the exposed 2x4 roof supports, find a skylight that will fit (the non-opening type are less expensive.) Find matching roof shingles (you won't need many), determine the flashing you intend to use, use your skylight as a template and draw the necessary cutout on the inside of the location. Drill whole from the bottom up through the roof shingles. Locate those holes and cut out the necessary opening, now use the skylight "footprint" to remove necessary shingles, find full shingles to remove. Mount the skylight and flash as per instructions, caulk where needed. Affix the new shingles. Test if necessary.