I did this DIY on my BMW 328i a couple of years ago and loved it (saved me a couple of hundred dollars the dealer wanted to install the factory buttons). So this is my version of someone else's DIY.
If you're sick of your garage door opener rattling around, or tired of searching for it as you approach your house, or just have a couple hours to waste, this one is for you:
1. a simple momentary switch (Radio Shack has a good selection...choose one you like but make sure it says "momentary" and is a push button).
2. a soldering iron and some skill with it
4. garage door and opener
5. 2-3 beers (I'm a lightweight)
As for the "step by step", not much to it.
1. Drink beer. Find a place on your dash to mount the thing. I like the space below my ash tray, since there are little pieces that come out for the express purpose of putting switches in there.
2. Disassemble your garage door opener and find the terminals that connect to the little switch button. You can do this by putting a piece of wire across them and if the door opens, voila! Solder about 6" worth of wire to each one, and cut a groove in the garage door case to feed the wires out. Reassemble your opener. Drink beer.
3. Drill a hole large enough for your switch, and mount that sucker in there. This step will vary depending on the switch you use. Consult your common sense during this procedure. Connect the wires (doesn't matter which one goes to which terminal) and test it out. Drink beer.
4. Make sure you can easily get to the garage door opener in the future to replace the battery. Also, wrap the thing in a cloth so it doesn't rattle around in there. You can also foam tape it to an inside the dash surface, but I didn't.
Alternate idea: wire the battery leads to some 12V wires in your dash. I did this in my BMW so I never have to replace batteries, but I didn't feel like doing it here. Plus, it's easy enough to get to the opener in my setup.