do i have to go onto the roof to fit a velux window?
Not if the model you install has a removable sash. You can simply stand inside and work your way around. As long as you can see and reach where the flashing meets the roof, you should be fine. The best way is from outside, to be safe.
If you want to do it the RIGHT way... you would/should go ONTO the roof in order to do it.
