Make sure the old sweatshirt will fit your dog.
How to Make a Dog Sweatshirt
Measure your dog from the base of the neck to midway to his tail.
Cut the sweatshirt to the correct length.
Either sew the hem or use fabric webbing.
Cut the sleeves so they hang just above the knee joint on your dog's front legs.
Hem the sleeves as well, if you prefer the tailored look.
Place your dog's legs in the sweatshirt first and then stretch the neck opening over his head.