This advice is specifically for the hips rather than the buttocks. If you go to the gym, use the hip abductor machine, high reps and low weight, gradually increase. This is the machine where you work the gluteus medius muscle by moving your legs apart against the weighted pads. If you’d prefer to work out at home, search “hip abductor bands”, they also work pretty well and you can use them sitting at a desk or something. They work by going above your knees and you stretch the band by moving your knees apart.

Not a fitness coach, or trans, just a guy who had been told he had a flat ass and wanted to prove em wrong. Happy with the results! In general if you want to feminize your body there are tutorials online about what exercises to do. Good luck!