When removing the sweatshirt, take it over your dog's head first and over his legs last. The neck hole will probably fit if you use a sweatshirt approximately the size of the dog.
Make sure the old sweatshirt will fit your dog.
Measure your dog from the base of the neck to midway to his tail.
Video of the Day
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.