Easy to do. Before you order one measure your roof thickness as there are 2 sizes of Heki (too accommadate the thickness of the roof.)
Remove old rooflight bedding mastic might be tough but sliding a warmer pallet knife between rooflight and the van will help.
Make sure you remove all old mastic and clean with a solvent to remove grease etc.
I used the pre made non setting mastic strips, dont use slicone mastic as this sets,fit Heki and tighten the screws equally as you go to make sure it sits square.