The tools you need for fitting a security chain or door chain can be bought below and you can click on the chains above to buy them.
There are two types of security chain. The simple version which allows you to open the door a little and the other which, while doing the same thing, acts as an extra precautionary door lock. Both are fitted the same way.
First hold the chain up to the door at the height you want it. Its usually just below the door lock. Mark the position of the four (sometimes 6) screw holes with a bradawl and use a drill bit which is smaller than the screws supplied to make pilot holes for your screws. Screw the slider to the door. Now mark the position of the chain holder on the frame. Make sure it is level with the slider on the door and not too far away from it so that the chain will not slot into the slider. Mark round the chain holder and chisel out any architrave to allow the chain holder to sit back flush with the door and chain holder.
Screw the chain holder in place and its finished.