How do I clean poop off my dog's butt?

First, of course, you would need to pick it up and set it in the trash.

Then in the store, you could buy some spray that makes the nasty smell go away. After spraying the floor, leave it for 5 minutes and clean it with a towel or a tissue. But if you do not want to buy the spray, then you could use wet wipes to clean the floor.