The first thing you'll need is someone who is comfortable around electronics. The following instructions are for a typical Microsoft Mouse. Different models and manufacturers will vary. The concepts are the same for Trackballs, and may sometimes actually be simpler. The purpose here is to end up with a modified mouse that is still usable as a normal mouse.
Find an old mouse and cut the tail off to be used for wire for the external switches. The normal mouse tail is nice and light and flexible and has 6 easy to work with conductors. We need 2 for each button, so we'll only be using 4.
Take the mouse, to be modified apart.
You'll find a screw or screws under the Teflon or the label.
Notice the Left and Right Micro-Switches for the Mouse Buttons. Remove wheel if there is one.
Remove circuit board from the Mouse.
Make a hole for the extra tail and run the new tail through it.
Flip Circuit board over.
Notice the Left and Right switch pins soldered to the Circuit board traces. This is where you solder your wires to for each external switch. Note: Some Mouse Button Switches have 3 pins. Only 2 are used. You'll see the ones soldered to separate circuit board traces. These will be the active contacts.
After soldering your switch wires to the Mouse Button Contacts, reassemble the mouse and secure the new tail to the normal one as a strain relief and for neatness.
The other end of the wire is your choice. We used 1/8" Mono Phono-Jacks from Radio Shack to plug our switches into.