I put a new pair of felt soles on a pair of boots 2 years ago and they are still holding up great. First, grind off all of the old sole so you have a smooth clean surface. Go to the hardware store and get a good quality contact cement, a foam paint brush and a roll of duct tape. Apply the contact liberally to the bottoms of the boots and to the felt...let them dry for about 15 minutes. Put the felt soles on the boots and wrap the whole boot tightly in duct tape, throw in the corner for a week and let them dry.After a week, untape them and they should be ready to go.