Step 6: The missing link
The front roller should be connected to the middle roller so that it in turn spins the front wheel.
I chose to use some 1" webbing, since I had some lying around. Lots of different things could be used for this however, rope, rubber tubing, rubber strapping, etc. The important thing is that you can get it tight, and so that it doesn't slip on the rollers.
It is easiest to figure out how long it needs to be by assembling the frame and rollers, and then measuring and marking directly. It is also better to make it a touch small so that it will be tight when you put it together
The belt is a single piece of webbing - with the two ends overlapped and joined together.
The joint is preferably sewn, but gluing should also work - see comments on pic