it is sorta a pain - however i've done it 8 or 10 times with three or four different 12" ibook G3 and G4's
follow the guide already posted - there are others available -just google 'upgrade ibook hard drive' or something
words of advice:
*really clear off your work space and the area around it- little screws can be hard to find if they fall
* there are different sizes of 'little' screws- try to keep track of what went where...etc- label them w/ a pen and what not?- i used to put different sizes on different plates/cups and put a label on the plate...etc
* relax first- this will take you about 2 hrs the first time
* well lit area
* magnetic screw driver can make it a bit easier
* close the windows first! turn off fans...etc - blowing around screws- not a good idea-
*the torx #8 driver can be purchased @ most hard ware stores- just ask for it- (no need to spend $15 or whatever buying it online)
really above all- slow down- don't rush it- and look @ how cool the stuff is inside your machine! this is your baby!