Source an undamaged roof section, not easy but they are out there.
Mark out the area you need to cover.
Cut out that section.
Keep the old section as you need this cavity to mark out and cut a sheet of aluminium to bond to your roof.
Mark and cut the aluminium sheet to cover the cavity, be generous either side of the cut.
Bond the sheet to the underside of your car roof.
Now from the underside drill holes all around the edge and using self tapping screws, screw and pull down the aluminium until it meets firmly with the roof fibreglass and the bonding adhesive is in full contact. Let the adhesive cure over night.
Now bond the cut out roof section to the aluminium.
Let that cure over night.
Now remove the screws and rivet the underside of the roof to secure the new section into place.
With the new section securely in place you can begin sanding.
The next process is simply filling and sanding until you find the contour, where the skill and talent comes in;)
Keep sanding filling until your are satisfied.
And there you have it.
Obviously my car will get a full respray (the black satin is temporary) as soon as I have finished off a few other body repairs.