Start with a MakerBot filament spool. Remove the label.
Take a printed seat and run a paperclip through the hole in one side.
Do the same on the other side, making large hooks to hang on the cross rod.
Add the cross rod by drilling holes in the spool and screwing the rod in place. Hook the seat on the rod.
Cut two supports, 125 mm wide at the base, at least 75 mm wide at the top. One is 127 mm high, the other is 117 mm high. They are cut from 1/2 inch plywood. Fasten a 75 mm by 25 mm piece of 1/2 inch plywood to the top of each support.
Add a continuous rotation servo to the taller base.
To convert a servo to continuous rotation, go here:
Insert a 120mm long 1/4 inch diameter rod in the axle cylinder.
The rod should be flush against the end of the cylinder where the two holes are located--the rod will push against the servo horn to keep it on the motor.
Secure the supports on a base (I used 12 inch by 12 inch 1/4 inch plexiglass).
Add the servo horn to the end of the axle cylinder.
Insert the cylinder through the MakerBot spool.
Push the other end of the axle rod into the bearing (vxb.com) on the bearing holder. Screw the bearing holder into the support.
Add riders (the frogs were made from clay by my wife). Provide 1.5 volts to the motor, and the action begins.