First, Walk it off:  Take it around the block a few times: Walk a block or two to warm up and then do 10 steps of walking lunges. Take short strides and walk slowly, dipping down as you walk, (bending your knee deeper than normal walking, but not too much-about 3 or 4 inches deeper). To protect your knees, don't dip to far. As your knee bends your bottom drops and you feel your rear supporting the movement on each side with each step. As you walk and dip slowly your butt will begin to burn, your quadriceps (thighs) may burn too, that's ok.

Keep it moving for 10 sets of 10-20 walking lunges with 20 steps of brisk walking in between.

Then: back inside, lie on the floor with your knees bent and feet on the floor. Tilt your pelvis up and contract your buttocks at the same time, then return your bottom to the floor and keep squeezing the cheeks. Up and down slowly, very small, very smooth movements, 2 sets of 20 repetitions. Then, keep it moving as you raise one leg to the ceiling and continue pulsing the pelvis and squeezing your butt (now supported by one leg and one cheek) for 10 repetitions on each leg.
Stretch it out: stay on your back, with one leg extended out in front of you on the floor, bending the other and bringing it into your chest. Hug that knee for 20 seconds and switch legs. (2-3 sets)

Do these exercises every other day, and  you will reduce your rear rapidly.