Step 1: Go get stuff
(x1) An over-sized tennis ball
(x1) AAA double battery holder
(x2) AAA batteries
(x1) Vibrating motor***
(x1) Small scrub brush
(x2) Zip ties
(x1) Cutting pliers
(x1) Razor blade
(x1) Coping saw or hacksaw (not pictured)
***My vibrating motor came from a back massager from Walgreens. You can learn to make your own here.
Step 2: Chop off the handle
The surface should now be made completely flat. Use your cutting pliers to trim off any remaining plastic. If that proves too difficult, you can cut off any plastic stubs with a coping saw (or hacksaw).
Step 3: Power switch
Place a small piece of paper between one end of a battery and the battery holder. This piece of paper will prevent the motor from turning on right away when we make the electrical connection in Step 5.
Step 4: Zip tie
Step 5: Wire it up
Next twist together the red wire with whatever color wire is remaining. This is typically a red wire, but in my case, the wire coming from the motor was blue.
The colors are not so important as a DC motor should typically be able to spin regardless of the orientation in which the positive and negative terminals from the batteries are connected.
(If the motor has no wires coming off of it, solder the red and black wires to its power lugs.)