Its rather simple. Unplug the IDE cables that connect the drives to the logicboard, then remove the screw that keeps the drive bay secured to the frame. Lift the drive bay towards you, away from the frame and remove the power cable from the drive. Then mount the drives into the drive bay (I'm assuming your machine has a dual drive bay) and use mounting srews to mount the new drives. Connect the power cables, slide the drive bay back into place, then reinsert the screw that secures the bay to the frame. Attach the IDE cables, and you are good to go.
One thing to keep in mind, make sure your boot drive is set to master (most drives show you where to place the jumper to do this) and that the other is setup as slave.
Here is a video from the apple support website that shows you how to do what I discribed above.
http://www.info.apple.com/usen/cip/html/g4ge/g4gbenet-atadrive-cip.mov.html
