I am an estimator and project manager for a commercial roofing contractor. Although I am in the commercial line of roofing and not the residential the procedures are virtually the same with shingle roofing.
The short answer to your question is no. It is not possible to do the job correctly without being on the roof.
First of all, the skylight needs to be anchored into the structure from the roof. You can probably get around that although I would not recommend it. Most importantly, it will be impossible to watertight the skylight without getting onto the roof.
Depending on how steep your roof is, you might want to call a professional. If your roof is a 6/12 or below you could probably do it yourself rather easily. Wear tennis shoes with good soles, walking up (and down) the hips and valleys, and along the ridge is easier, you can rent or buy a safety harness with a rope slide to prevent you from falling off the roof (or prevent you from hitting the ground if you are working near the edge). However, you must get to a proper location and install a tie off before you can ?hook in? your safety harness. The best place for a tie off is near or over the ridge because it gives you the best range of work. You NEVER want your tie off location to be below you (as in nearer the edge of the roof than where you will be working or moving). Have the person you are getting the harness from instruct you in the proper way to wear and use it. Do not leave until you are sure that you can put it on properly. A harness that is worn incorrectly could (if you fall) put a few thousand pounds of pressure directly onto your balls. Never work alone even if you are wearing a harness. You have a limited amount of time that you can hang in a harness before the circulation being cut off in your legs can lead to permanent damage.
If the slope of your roof is greater than a 6 on 12 I would suggest hiring a pro.