I suggest this to everyone who gets a used industrial sewing machine. Don't try to do this yourself, changing that belt is not easy. Take it to a professional sewing machine mechanic, have him set the timing, set the tensions, and check the machine out for worn or broken parts. Have him put the belt on at the same time. It will be the best money you ever spent. That way you start out with the machine sewing like it should right from the git-go. Also, if you got any thread with it use that thread for practice only. Why ruin a project by using bad thread?
If you really want to ignore this advice and try to do it yourself, check out page 12 of this service manual: http://parts.singerco.com/IPsvcManuals/211W155.pdf
This is a 211W and not a 111W, but you'll get the idea. If you don't do this correctly, the machine will be out of time and then you have a whole new problem.