To be honest, the instructions weren't massively clear, but with a little common sense, it's easy.
Remove headlight.
Remove the plastic cover off the back of the light and drill a hole in it to fit the rubber grommet.
Put through the hole the bulb and plug socket.
Fit bulb in and connect plug socket to plug on lighting circuit** (the plug the original bulb connected to).
Push rubber grommet in hole.
Put the rear plastic back on.
Connect the all the small plugs up and large plug for the ballast.
Put light back in, connecting it back up to the car!
** This bit needs a little common sense. The plug which connects to the original bulb, has a clip which goes over a little tab on the bulb, so looking at it, you know which way to connect it up.
On the HID kit, the plug socket which connects to the plug on the vehicle wiring has a tab on each side, so you could potentially connect it to the car wiring the wrong way round. Make sure you check the instructions and the vehicle wiring to ensure you know which wires are positive and negative!
If you buy the kit, all will become clear!