*Note* This is the install that I did for my particular HID conversion kit. Your results may vary.
My HID conversion kit came with 2 ballasts with built-in ignitor, and 2 bulbs.

1) Take all safety precautions. Disconnect battery, leave bike in off but do not lock the steering.
2) Following the instructions in the owner's manual, I removed the front fairing so that I would have more room to install the bulbs and ballast.
3) Undo harness from lights, remove rubber dounut from head lights, unclip the metal clip holding the light bulb and remove the old bulbs. Install HID bulbs in reverse, but becareful not to touch the bulb itself as the oil from your fingers will heat up when the light is on and cause the bulb to break.
4) With some of the body work off, decide on where to place your ballast and ignitor units.
The ballast and ignitor units from my kit fit in between the headlights and dash. In between the headlight and dash are two relays that are connected to the headlight housing by some rubber clips. I took the relays off and taped them to the back of one of the ballast and had the wires from the ballast run up where the relays use to be. Depending on how big your ballast are, there should be plenty of room there for two ballast.

I drilled two small holes into the fairing and zap strapped the bottom of the ballast to the fairing and zap strapped the top of the ballast to some of the wires.

5) I wrapped all the wires in electrical tape and tried to hide all the wires along the factory wires. Connect the wires from the bulb to the ballast. Connect the power wires from the ballast to the harness. Remember, ballast red to the harness blue and ballast black to harness green. IIRC low beam is blue with white stripe while high beam is solid blue.
6) While I had the bike apart, I removed the two black boxes from the intake tubes and covered up the holes with tape. With the added space, that was where I hid the stock harness plug and ballast connection. I taped up the plug with electrical tape to prevent water leaking into the plug.
7) Put bike back together in reverse order.
Finished product.
02 Sentra SE-R Spec V
92 NX2000
02 F4i