It shouldn't be too hard and I'd suggest that you head to your local Radio Shack, they're cheaper than most hardware stores. For housing, you can be as creative as you want. I've ordered super bright LED strobes and an LED strip from Hong Kong to create my own turn signals and brake light. I got a center off toggle from Radio Shack for the turn signals and just finished creating the housing for it. Since it will be handlebar-mounted, I went with something barely bigger than the toggle switch itself--the bottom 1/3rd of a vitamin bottle did nicely. After cutting the plastic bottle, I had to drill out a fairly large hole in the "top" of this housing to accommodate the toggle and then smaller holes closer to the "bottom" for zip ties that will hold it on the handlebars. It's now spray painted flat black and should blend into my black handlebars. (I will glue insulating foam to the bottom of this once I've got the toggle switch wired and installed in the housing. The foam will contour to the handlebar and ensure there's less chance for movement when I use the toggle switch.)
Yes, quite easily. It depends on where you put the batteries in...front or rear. You would however need a little soldering skill. You can pick up a cheap soldering iron from a discount store in some cases. In a pinch you could probably use some tape instead.
Basically, where the battery touches the head of the flashlight in the front, or the battery spring in the rear (Mag Lights for example) has to be separated and you run a wire to each and put a toggle switch between. I used to take apart flashlights as a kid and use toggle switches off my toys and rig up stuff like that.
Electrically it's really basic. The remote switch is wired in parallel with the flashlight switch. When teh flashlight is off, the remote switch has the capacity to control the light. Turn the flashlight on and the position of the remote switch won't matter.