A security chain allows you to open the door slightly to identify and talk to visitors, and gives your door some reinforcement when closed.

[edit] Steps

Choose a height for your security chain and mark it on the door. Make two marks so it is level.
Select a screw head drill bit and attach it to the drill. You will be using this to attach the slide channel to the door.
Align the slide channel with your pencil marks and drill the screws into position.
Use your pencil to mark the position of the slide channel on the door frame. Hold the chain holder over your pencil marks and draw around it. You will need this outline in order to chisel out a recess.
Make a series of deep cuts along your pencil outline.
Remove the wood.
Hold the chain holder in the recess and drill it into place.

[edit] Tips

Look at the diagrams in the instruction manual when in doubt.
Before you start drilling, remember drill safety. Put on your safety goggles. Make sure that all loose items of clothing, and hair are tucked away.