Learning how to do donkey kicks.
'If you're searching for an exercise to target and tighten your rear end, look no further than the donkey kick. ... The donkey kick's scientific name is a quadruped bent-knee hip extension. But it gets its nickname from the literal movement, which looks like the animal's notorious kick.'
There are some excellent YouTube videos available to help you get the hang of it.
Here's one:
https://www.youtube.com/watch?v=Im5nE3J558k