Make sure you have proper clothing on (long sleeve shirt, long pants, a mask) because insulation is terribly itchy and not good to breath in either.

What to Bring

In addition to proper clothing, bring a framing square, the recommended hole dimensions of the skylight, a tape measure, pencil, hammer, and some nails. Also bring a plumb bob (You can just use a piece of string that will reach from the roof joists to the ceiling joists (6' or so) and tie a washer to one end.)
Again I'm going to request that you use your imagination to invision the skylight in the ceiling. Now, go straight up from there and place your plumb bob up against the inside of one of the ceiling joists, just above where your wire comes through the ceiling. Let the weighted end hang and move it around until it is directly over where the wire comes through. You'll want to place a nail in that location and drive it up through the roof.
This procedure is assuming the skylight was designed to fit a standard ceiling joist spacing. If your skylight spans more openings, you'll have to do a bit more work while framing the opening.
Using your framing square, lay the short leg against the joist and mark along the long leg. Flip the square around and place the short leg up against the opposing joist. Line it up with your line and continue drawing the line.
The next step depends somewhat on what type of light you're putting in and what your directions say. If your skylight will have a curb, and you have the outside dimensions of the width of the curb, deduct twice the thickness of the curb. Otherwise just use the inside dimensions. That will be the measurement you use for the width of the hole. If the skylight is smaller than the distance between the two rafters, you'll want to put it up against one of the rafters and measure out from that one. If the light is fairly close to the distance between the rafters, you'll just center it. Repeat the same procedure at the other end to get yourself a line for the other end of the hole. You should now have two parralel lines that denote the ends of the skylight.