Old Craftsman lawn tractor has started shredding belts. The new belt came apart after one hour losing the narrow part of the V. The next new belt is wearing on the narrow flat surface after 10 min use; smell burning rubber. Idler pulleys seem to move easily. What could be wrong?

Spin your pulleys if they make any noise .sounding like a grinding noise or if they feel like they have gravel in them replace them.
Make sure your getting the belt on the correct side of the belt guides.