sit almost like you are going to be cross legged on the floor, but without letting your legs cross.. i.e. if stretching left butt, cross your legs with your left leg below and right foot over your left knee. try to keep your left shin as parallel as possible. try to bend forward a little and take deep breaths.. you should feel the stretch on your left butt muscle. If tummy is getting big, then you can bend less. but just try to stretch forward and repeat with the right butt by swapping your legs (i.e. right leg below with left knee on right foot).
try and let me know if it works for you. hold for a few seconds and repeat.