Step 1: Parts is parts!
You also need about 4 feet of 3/8" vinal tubing. Home Depot sells this in the plumbing isle, but only in 10' rolls. A roll is less than two bucks, so you aren't breaking the bank. Keep the rest of the hose so you can replace it when little black moldy things start growing in it. At $.75 you can toss it often, whereas a Camelback user has to run a scrub brush down theirs. Sounds like a fun Saturday night to me!
You'll also need to keep the bite valve from your previous system. You can buy replacement valves wherever the original was sold. I'm currently working on designing a bite valve from silicone sealant, for the truly cheap.
Step 2: Put it all together
Shove the tube through the hole, it should be tight. You can seall it with hot glue, but I didn't. I figured it would allow a little air to back flow into the bottle as I drink. It turns out that almost no air leaks through, but it works well enough without the hot glue. Glue would totally seal it, however.
Step 3: Drink Up!
*You can stuff it in any pack
*You can fill it from a stream (use tabs)
*You never have to wash it, it's disposible!
*The tubing is cheap enough to replace
*There is no leaky plumbing under water in your pack.
Step 4: Updates
I stuck the tubing end into the pull-cap and hot glued it in place. After several rides, I can say that it works GREAT, no leakage at all! Plus it was free.
The inside end of the tubing would get stuck to the bottle wall by my suction. I cut the end of the tube to a V and also cut a hole in the side of the tube, about 1/4" from the end. This keeps the tube from being sealed off by contact with the bottle.
I also made a hydration pack from some backpack parts I had laying around. (from a daypack on it's way to the trash). Looks professional, Someday I'll post it too.