Treadle belt is pretty easy and instructions sometimes come with belt. Here's what I'd do if I didn't have the 3n1 treadle pliers:
---Get proper size belt in natural leather (stay away from red) from SM supply, ebay or.....
---wrap belt around treadle pulley and SM pulley and pull tight and cut where ends meet (don't worry about cutting short the belt will stretch)
---examine wire staple that comes with belt, get drill bit slightly bigger than staple diameter and drill hole in each end of belt half the width of staple (so holes are width of staple when belt is put together)
---Put staple thru one end of belt and crimp tight on other end leave alone.
---Put belt on machine with uncrimped staple end facing up and run treadle for a few minutes
---Belt will stretch and cut off belt at hole at uncrimped end and drill another hole half width of staple
---might have to do this 2-3 times till belt is broken in
---crimp second end when belt seems stable and belt doesn't slip but not overly tight
---imagine Youtube and google would also have info on this.
Good Luck, it sort of a fun process