First, remove all the bits from inside your victim/mouse of choice.
My dead mouse had a screw on its belly. Removing this one screw let me prize it apart to expose its innards. I recommend you do this project with a computer mouse, not Canida
's Mouse Mouse
, real mice don't agree with magnets and their fur would get awfully mucky with whiteboard pens rubbing on their tummy.
Unplug the USB lead. Different mice might not be made with the same easy connector as mine, so if necessary, clip it off with some wire snips. (It may look cool with the wire snipped an inch from the input with the wires frayed, a patch as a prize to anyone that sends me a photo of theirs!)
Lift out the PCB and return the mouse wheel to the cradle it was in. Again, different mice will have different construction so you may need to keep part of the PCB in place so that your wheel will stay in the right place once finished. Put the PCB aside, it has loads of useful switches and other components which would be good for an electronics project later.