Bring the bottom pieces together like you are going to button them and cut a hole for their tail and backside. Once the hole is made, button the onesie closed. If the neck is loose, you can use a rubber band to bunch the material together. To fit: Put your pet’s back legs through the leg holes in the shorts. More @Wikipedia
Hover over any link to get a description of the article. Please note that search keywords are sometimes hidden within the full article and don't appear in the description or title.