The following page of the Final Fantasy 7 Remake guide describes the Burning Thigs side mission in Chapter 9 - The Town that Never Sleeps.
This side quest revolves around the squats mini-game. The rules are simple - you have to press the right button when the arrow stops on them. At first, you will be shown a diagram with the buttons, but as you catch your rhythm, you will have to press them without looking. It's best to remember Cloud's movement and take your time pressing the buttons. We always follow the same rhythm - Triangle, Circle, Cross, Square. Over time, you can push the buttons more and more quickly, so if you're already "getting better" at squats, just push the other buttons and you'll get a large score very quickly.
The first round is very easy to win and requires approx. 13 squats. This way you complete the task and receive 3 mega-potions. However, it is worth continuing because there is one of the better items in the game waiting for you. At the amateur difficulty level you have to do around 25 squats and you'll get Lucky Up Matter. You're going to face the champion in the final - you have to do around 43 squats to win. The prize, however, is the Champion Belt, which gives +10% to Max HP and 5% to Strength-perfect for Cloud and Barrett!