(note: these instructions were made after the actual device, so the pictures are of the finished product disassembled)
Step 1: Preparation...
1 old joystick (just the handle/trigger thing)
an old optical mouse
soldering iron (you can build it without soldering, but it's not as reliable)
hot glue, duct tape, third hand, molten plastic or whatever to hold it together.
Step 2: Take Them Apart!
in this case, the trigger wire is red,
the top button is white,
and common is blue.
(common is a wire that connects both, so you only need 3 wires instead of 4)
Step 3: Mod the Mouse
i chose to use the trigger (red wire) for the left button, and the top button (white wire) for right.
find the common terminal by following the circuit traces, there will (should) be one wire/trace going to both (in this case 3) mouse buttons, this is where you will attach the common wire.
On my, and most other mice, the common is the ground. (the negative power source everything connects to)
Step 4: Physically Attatch the Mouse to the Joystick.
but, If you do happen to have the exact same mouse and joystick you can hammer a pen onto the screw anchor thing, and use the joystick's screws like a vice to clamp the joystick to the pen.