Many people do not realise that it is perfectly feasible to fit a Velux roof window without once ever having to step outside onto the roof !
During 40 years of house restorations/renovations, I have done several like this and it really is quite easy:
If there is a sloping ceiling already in place where you want the Velux, first cut an opening in this between rafters, just big enough for you to be able to stand and work in, say 24" x 16". If no ceiling present, so much the better. Cut away an equivalent area of the sarking felt with a Stanley, also.
If the roof is tiled, just release a few tiles over this same 24" x 16" area by prising them over the battens. If roof is slated, you will need to cut the battens each side of the 24" x 16" area with a jigsaw and some slates will come away with them, as slates are nailed, whereas tiles generally aren't.
You now have a 24" x 16" actual hole in the roof. Be reasonably sure of fine weather because the roof will be open for about 5 hours if you take your time with the job. (An old boss of mine once said to me: "Only two ways to work - quickly or properly.)
Make sure you have a working platform with trestles or bandstands such that you can stand up with the bottom of the opening at around waist height.
Now, work around the opening, removing tiles/slates and exposing battens over slightly more than the area of the Velux window. Try to ensure that the lowest course removed leaves tops of tiles/slates about an inch from the intended base of the Velux.
Cut away unwanted felt/battens as you work, taking great care not to cut any greater width out than the Velux width + 2 inches.
There will likely be a stonking great rafter halfway across your opening, which needs to be cut back and trimmed, as per the normal Velux instructions.
Everything from here on is as per the directions, except that you will be working from the inside, through the Velux outer frame, not balanced precariously on your roof !