ollie1shoe,
Treadle belts need to be just tight enough to not slip, but no tighter. I slip the belt on the wheels, through the guides and pull the end against the long part. Then cut it 1" shorter. Leather belts will stretch so if you can pre-stretch it you'll be better off.
Sew-Classic has instructions on how to instal treadle belts that say it better than I can.
Joe