Redditor MCHammerCurls (shout-out to xxfitness) bought a lovely new pair of leggings from Celestial Bodiez in which to hit the gym. These leggings are specifically designed for female gym goers with the glutes to prove they spend time in the Iron Temple. Given their target audience, Celestial Bodiez provides a useful guide along with the leggings for taking not one but two types of butt selfies.
For this first selfie, the guide recommends to "focus on really pushing hip towards the mirror" with one leg carrying all your weight. Then you "take a backward step with the opposite foot." Only then should you "place your non-selfie hand on the front of your thigh on the planted leg." An added measure is to "give a little tricep flex."
For the second butt selfie, you should stand at a 45 degree angle from the mirror. The guide is very clear about this. Then there are some complicated instructions about popping your hips at the mirror, followed by a knee bend with your other leg. Again, one must lay the spare hand on the thigh closest to the mirror. Finally, after some back arching, you have completed the pose. "Smile. Selfie," the instructions finish.