Swivel snap broken. With a hacksaw, cut the loop that fastens the snap to the leash. Buy a replacement snap at hardware store. Slip the loop through the end of the leash and hammer it closed.
Leather handle broken. Cut the broken ends off cleanly. Apply Pliobond cement to one side of one end and the other side of the other end. Let dry until tacky. Then overlap the ends about 1" and press together. When glue has dried, punch a hole through the overlap and insert a steel rivet.
Leather leash damaged. If badly weakened, cut out the bad leather, overlap the cut ends and glue and rivet together as above. If leash is simply delaminated, spread Pliobond cement on the laminations and press together.
Chain broken. See
chain.