It just takes some custom work, involving cutting holes for the bigger HID bulbs, and then running all the wires to the ballasts and mounting them. There isnt really a set way to do it, and I havent seen a tutorial around I dont belive.
Because the HID bulb is so much larger you will have to cut the back cover of the foglight and then develop a way to seal them back up so theyre water tight. Other than that its just plug and play with the hids, as far as I know.
I should note, I havent ever done this, I wanted too but decided it was too much work for what it was worth. You could alternatively try yellow halogen bulbs, inside your (i assume) yellow lens lights.