I don't know of a guide, but if you've done it on a pc before I'm sure you won't have a problem. I've had to install Airport cards into iMacs before and it just looks like a really nice pc inside. The hard drive looks fairly accessible but I wasn't looking at it that closely. Just open it up and have a look.
Take the computer and place is screen down on a towel or something soft on a table. Then unscrew 3 (pretty sure it's 3) screws on the bottom. Then you can lift the cover off by the stand. Bottom comes up first then off the "clips" at the top. Then you will be able to see how easy it is. Put it back together in the opposite way. Clips, bottom, screws.
Good Luck