Putting on a treadle sewing machine belt is a pretty easy endeavor. First you acquire a leather treadle belt. It should be about 72 inches in length and have a heavy duty staple on one end. You can find these on the internet for sale. I also have them available in my Etsy shop. If you have a belt with no staple or lost the staple you can use heavy button thread and sew it on. I’ve had to do that and it holds just fine.
Hand wheel with belt groove
If you don’t have a belt and are putting one on for the first time then you need to identify the track where the belt goes. On the top hand wheel you will see a groove.
Fly wheel with belt groove
Next you will identify the fly wheel underneath the machine and the belt groove in it.
Fly wheel with belt
Feed the belt into the belt groove of the fly wheel and the hand wheel, holding the ends together in front of the hand wheel.
Hold ends together
Make sure to pull the belt as tight as it can go.
Snip the end without the staple at an even length with the other end (the end with the staple in it).
Make a hole in the belt
Make a hole in the belt about a quarter inch from the end. You can use a small nail or an ice pick. Make sure the hole goes through the middle of the belt.
Attach the ends of the belt
Attach the belt through the staple and close the staple with a pair of pliers.
If you use the machine a lot you may have to shorten it in a month or so. If the leather stretches and you feel your machine slipping when you treadle, that’s when you will know it needs to be shortened. Then you open one end of the staple and proceed with a new cut and hole as above. 😀
Sewing With Treadles 
Your post was very informative thank you for that
I’m so glad and you’re very welcome 😀
Thanks for the great info. I have 3 treadle machines and the belt came off one and I forgot how to do it. Now to get the belts on the other two machines and I will be soooo Happy!!!
That’s great! Happy treadling 🙂