Ok I'll try and explain without use of pics (it's too late and dark to take any lol)
Basically in hindsight I should have taken the light units out to fit them properly but with the cold weather and little time on my hands, I just couldn't be ARSED haha so mine is an initial slight botch job which will be corrected at a later point!! But I still managed to get it all up and running without taking lamps out so it proves the job is not that difficult at all.
The low beams are H1 so obviously your HID kit will also be for H1 fitment. The HID kit should come with xenon bulb which wires to the ballast and this then wires to two open spade connectors (on my kit one is red wired and the other is black). So all you have to do is remove the plastic grey cover cap behind the low beam bulb on the car, pull out the two black plastic clips which connect to the current H1 bulb and then remove the bulb itself. Once this is done, you just plug the RED spade connector from the HID kit into the plastic socket with the GREEN wire (from original S15 loom). The connect the BLACK spade connector from the HID kit into the plastic socket with the BLACK wire (also from original S15 loom).
You'll find the OEM plastic socket for the black wire is smaller than the one for the green wire so the HID black spade connector does not push all the way in?! It'll go in a bit but it's not tight so in any case, I've used lots of insulating tape around where the spade connectors have been pushed into the OEM black plastic sockets. Last thing you want is a dodgy connection!! Tom (sliding-r) recommended that I solder these wires but I'll do that at a later point
So to recap you should have the following setup once the HID kit is fitted together:
ORIGINAL LOOM (GREEN + BLACK WIRES WITH BLACK PLASTIC SOCKET ENDS) -> HID KIT SPADE CONNECTORS (RED + BLACK) -> BALLAST -> XENON BULB
Stick the xenon bulb in low beam housing and that's it!! Ok to do it properly you need to pull the lights out and tidy away the wires so it uses the hole that had the original loom grommet etc so everything except the ballast is not showing. My version was to let all the wires remain outside of the light housing and then I used loads of insulation tape to put the grey plastic cover back on but with HID kit wires coming out of it to go to/from the ballast there's not a proper seal anymore. Must remember not to drive through torrential rain haha!!!
Oh well, when I have more time...
J