Fix the booster packs down. I intend to fix them inside the headlight box on my GTD, bolting to the side through the fibre glass. I most likely will cable tie the wiring loom to the hinge bar holding the front clip on (maybe no good if you have a quick release front clip!) making sure the wires can't get chaffed. Also plan to use grommets to seal up the holes I make into the headlight boxes.
So now in theory the boosters and relays are fixed down, the wiring loom connected and tied on, so it just remains to replace your existing H4 bulbs with the Bi Xenon ones, make the final connections and check to see if it all works as it should!
I say in theory as I have yet to do this to my car. Currently my Red Top 60 battery is dead and is having a post mortem examination at DMS Technologies to see why. I hope to have it (or a new one) back in about a week or so and then will be able to put the above into practice.
Of course, in the mean time, if anyone spots any erros in what I have said, or has any other suggestions please do post your thoughts here.