How to install a catflap in a steel door
Step 1: Get a Dremel.
Step 2: Get a Cut-off wheel and a grinding stone. Grab a steak knife from the kitchen.
Step 3: Sketch the template outline on the door in accordance with the instructions.
Step 4: Cut down the outline you have drawn, using the Dremel and cut-off wheel, just through the steel. Wear safety glasses for this, really. The sparks fly!
Step 5: Pry out the steel. It's just an outer lining. Take the steak knife and carve out the foam inside the door. Smooth the steel edges with the grinding stone.
Step 6: Cut out the other steel lining with the cut-off wheel. Smooth out with the grinding stone.
Step 7: Shove the cat through the hole a few times.
Step 8: Follow the instructions for putting up the door. Here, you will need a metal-cutting drill bit and probably a regular drill, or maybe an attachment for the Dremel. Anyway, my bit wouldn't fit my Dremel. Do not drill into the foam; you only want to pierce the steel. Your cat should catch on to using the door right away, if it's been through the open hole a few times. If your cat is dumb, like one of mine is, put them on one side of the door, and food on the other. Go shopping. By the time you get back, your cat will have figured it out.
The above is the wrong, Persnickety way to do it, but darn it it worked. The right way is to draw your outline, use your metal bit and electric drill to drill holes clear through the door in each corner of the outline. Then, using a jig saw and metal-cutting blade, insert the blade into one of the holes and cut out the door hole clean through, following merrily along the outline. Power tools and $10 blades notwithstanding, I lacked the physical strength to control the saw, particularly when it cut through the steel, encountered the soft foam, then hit hard steel again. I actually broke the saw blade off inside the door. The right way is probably the quicker way, for those with the muscle. But the right way is the wrong way if it's not personally possible.
P.S. You may have another bit of difficulty with the wood at the bottom of the door, or the door panels. The bottom two inches of my door is wood. My cat flap just barely fit. Ideally, the bottom of the cat flap should be at the same height as the bottom of your standing cat's belly. You might want to do a little measuring - of the cat, the door, and the various available cat flaps, before you start carving holes in anything.