Place the skylight dome where you want it and work out how many tiles you need to remove. After removing the tiles, place the dome on your chosen spot and outline it, to mark the tiles that need cutting.
Before installing the dome pull the Mylar tube over the back of it and use the silver tape to secure it. Put the side marked ‘top' closest to the top of the roof. Feed the Mylar tube into the roof space and put the skylight dome in place by tucking it under the surrounding tiles.
Use silicone to waterproof under the side of the dome. Make sure you apply a fair amount of silicone under to prevent any leaks in the future. Now use a hammer to gently shape the flashing around the dome to match the roof's profile. Then apply more silicone around the flashing to seal it.
Before you can cut a hole in the ceiling, you need to know where the joists are. You can use a stud finder for this. The ideal place for the skylight diffuser is between these joists. Once you have selected your position, measure across the width of the ceiling and mark out a centre line.
Use the diffuser as a template to mark out where you want the skylight to be. Then using your plaster saw, cut out the hole. Next, pull the Mylar tubing from the roof space, through the hole. Make sure it's the right length. You may need to trim it if necessary.
Fit the diffuser into the hole, using a drill to fix it into the ceiling. Screw it all the way through, but you need to secure it into the plaster using the plastic tabs over the top of the screws. Tighten the screws further for a nice firm fit, then insert the screw caps.