How do you make one? I have a spare bike and a trampoline. I want one to screw around on.

take off front wheel, back wheel and cranks.
put foam on the bottom of the forks, on the rear dropouts, and any sharp areas (barends, etc)
tramp bikes are mega fun :)

My one friend said he flattened a basketball and put that on the front with lots of duct tape and flatened a football to put on the back with tape. I wonder how that would work?

. . .you flatten it then duct tape it to the dropouts, etc