Here's a list of supplies you'll need.
7" Hamster Ball
High Torque Metal Gear Servo. I used the HS-645MG
Aluminum Servo Shaft Horn 250HS
(3) Aluminum Hubs 3463H
RZ12-90-128RPM Gear Motor
Gear Motor Mount
(2) 1/8 ID Bearings
1/ 4 ID Bearing
Any Hobby Shop:
2 channel radio and receiver set (More channels are fine)
(8) Elite 1500 batteries
Cheap ESC (Electronic Speed Control)
3/ 4" Aluminum Flat Stock
3/ 4" Steel Flat Stock
Misc Nuts and Bolts
Double Stick Tape
Lead weights (Or steel)
Great Stuff Expanding Foam
Step 1: Sketch / Plans for the Ball
Step 2: Attach Drive Hub To Ball
Step 3: Make the Chassis
Take a look at all the pics and you'll get a good idea of what you need to do.
Attach another one of the aluminum hubs to the gear motor drive shaft. Screw some bolts into it and sand them down so they fit nicely in the hub that you just mounted to the ball.