Cut the suspenders the point where the shoulder straps meet at the center of the back. This leaves behind two adjustable straps and a shorter length of material attached to one buckle, sometimes two, depending on the design.
Take one of the shoulder straps, and put the rest of the material to one side for now. If the suspenders have clasps, cut the strap just above the clasp. If the suspender is designed to fit onto a button, it will have a buttonhole at the base. In which case leave the buttonhole in place. Ensure you don’t trim off or damage the adjustment buckle. You now have a single length of material, partly looped through a buckle for height adjustment.
Make a 1-inch slit in the center of the suspender, approximately 2 inches from the end. If you trimmed of the clasp, make a second slice at the other end. If the other end already has a buttonhole, don’t make a slit.
Put a thin layer of glue on the back of the rubber washer from a beer bottle. Glue the back of the washer over the slit. The rubber washer reinforces the slit and bears much of the tension from the weight of the guitar. If you make a second slit in the other end of the suspender, glue a second bottle washer over the slit.
Fit the suspender over the strap button, so the strap is sandwiched between the guitar and the washer. The buckle end of the suspender should go over the bottom strap button.
Fit the other end of the suspender over the top strap button. Brace the guitar with your knee, and adjust the buckle, so the guitar sits at a comfortable height for you to play.