If the white plastic scissor bracket is off the keyboard, separate it from the key. The scissor bracket is composed of two parts that snap together, one inside the other. If necessary snap the smaller bracket into the larger bracket, two plastic pegs on the smaller should find matching holes.

The bracket then should be put onto the keyboard around the key post, using the least amount of force to get it seated. There are three contact points, a flatter metal tab and two vertical metal loops. Hook the bar of the smaller bracket under the metal tab, then two pins on the larger bracket should catch between the vertical metal loops.

Last line up the key cap with the key post and the scissor bracket. If all is lined up, just a little more force than normal typing should snap it back on.

I recommend this site for further reading and the good photos.

http://www.nonverbla.de/blog/2006/11/12/how-to-clean-your-macbook-pro-keyboard/

I’ve also read on this blog:

Also, if you are able to go to the Genius Bar at an Apple Store, they generally have stashes of old, dead keyboards and can give you replacement keys and scissor-brackets for free, no questions asked.

I’ve also heard that the level of competency and service can vary quite a bit between Apple stores. There are two near me, and another Apple user I know says the more distant store will fix something in 5 minutes for free while the other wants to write up a service order and says they should get to it by next week.

As I was looking for a set of instructions for placing the shift key back on my MacBook Pro, I came across this page. I notice that others here had the same issue I did: the proper technique for replacing keys. I found an article that layed the procedure out perfectly. I hope it will be of use to someone else:

http://www.powerbookmedic.com/MacBook-Pro-Keys—Individual-Key-Keycap-p-16753.html