Very easy - I've just done this.Take off the battery and there are I think 2 very small screws (Philips size 0, but a knife might work). Take off the RAM cover and press in the new RAM module. Then close up and try About This Mac to check it is recognised.
All takes about 10 minutes.
BTW, if you have not bought the RAM yet, Apple is the most expensive place to get it. I got mine from crucial.com.
Good luck