Unscrew all screws in the back of the tablet. If you think you have them all but the tablet still won't come apart, check under stickers!
Very carefully pry the casing apart (using a flat screwdriver to gradually pry up the edges helps). If you're using the tablet I did (the Wacom "Bamboo Fun") you can pull out the scroll touchpad from a little plastic slot but BE CAREFUL! I damaged the connections when pulling mine out and ended up breaking the touchpad. Carefully remove the board and the insulating metallic sheet behind it.
I really ought to have more pictures for this step, but I disassembled mine a long time ago. It's pretty straightforward, but let me know if I can offer any assistance.