Looking to firm your butt please see the exercisers below that claim to be the best at doing this I think the squats achieve the best results.
Stand with your feet shoulder width apart and with your hips and keens in line. Slightly contract your abdonimals to stabilise your torso and spine and slowly bend the knees and lower your body as if you are going to sit down on a chair. Lower your bottom until your thighs are parallel to the ground. Slowly return to the standing position and repeat 15 times.
Quadruped Hip Extensions
On your hands and knees for this one and slightly contract the abs again to stabilise the torso and spine. Lift one leg up keep the knee bent at 90 degrees. Lift the leg until th ebottom of your foot is pointing towards the ceiling. Repeat on the oppisite leg do between 8 and 12 reps per leg.
Stand with good posture at the bottom of stairs hold a dunbell or a tin of baked beans in each hand. Place the left foot on the step and transfer your weight to that leg pushing down in the heel to come up on top of that step. Keep th eright leg passive repeat about 8-12 tomes and change legs.
Single Leg Squat
Stand on a box or bench with all your weight bearing on one leg and let the other leg hang of the side. Bend the weight bearing knee and lower while pushing the hips back. Push down through the heel to come up. Repeat on both legs between 8 and 12 times.