Petsafe Big Cat Flap Guide D'installation page 6

Cut the opening
1
If desired, remove your door from its hinges and place it on a
raised, level surface, such as two sawhorses. The interior side
of the door should face up.
Removing the door from its hinges makes it easier to cut
the opening for the cat flap, but you can install the cat flap
without removing your door from its hinges if you prefer.
Important: Make sure there is nothing behind or underneath
the door where you will cut the opening.
6
2
Drill 3 mm (1/8-inch) pilot holes at the centre of the
13 mm (1/2-inch) circles just inside the corners of the cutting
line. These pilot holes will help centre the drill bit for the
subsequent larger holes; they do not need to go all the way
through the door.
Switch to the 13 mm (1/2-inch) drill bit and, centring the
drill bit in the pilot holes you just drilled, drill holes all the
way through the door. Hold the drill steady and straight at a
90-degree angle to the door.
These larger holes will be the pilot holes for the jigsaw blade.