Making the opening
Mark out the opening on the underside of the roof membrane. Do not remove more than necessary - you can always cut it back more later.
Removing the membrane
Cut out and remove the membrane, exposing the battens and the roof covering.
If you have a tiled roof, carefully begin to remove the tiles from inside. Make sure the area below this roof area is clear to avoid injury or damage if an accident occurs.
Composite thin tile covering
If you have a composite thin tile covering, you may need to drill and cut through to make the initial opening.
If you have a slate covering it may be necessary to use a slate ripper to remove the nails.
Remove sufficient tiles/slates to allow a good working area. Don't be tempted to remove too many though - you'll have to put them back later!
Remove battens to the extent of the framed area.