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.