Mark the center point on your door and determine how high on the door that you want the peephole to be.
I would recommend criss crossing some blue painter's tape over the point to protect your door in case the bit slips.
Put your safety glasses on and pre-drill a starter hole through the door using a smaller drill bit for steel. Then switch to the correct size steel drill bit and make sure that the bit is level. Drill the hole halfway through the door from both sides to prevent splintering.
Then use some steel wool or a metal file to clean up the edges and then remove the tape.
Push the unit through each side of the door and screw the pieces securely together. That's all it takes to give yourself a some peace of mind!
Just check your directions before you do any drilling. I'm Mrs. Fixit and it's just that simple!