SQUATS
How it's done?
You have couple of options to to do it, with free weights, dumbells or with a smith machine. The smith machine isn't explained here, since its almost the same butt exercise as barbell version, only thing is that the barbell isn't "free".
With barbell
In the gym you'll find a rack which has a barbell mounted on it. Go stand under that barbell and and position the barbell on back the of your shoulders and grasp the barbell with your hands. Now you can dismount barbell from the rack. Your feet should be approximately at your shoulder width stance. The executing of the movement happens by bending your knee and descending until your thighs are parallel to floor, then start extending your knees and go back up a gain. Now you have done one repetition.
With dumbbells
Take weights in your hands on your both sides, stand straight and descend until your thighs are parallel with floor. Then go up again. Now you have done one repetition.
Things to consider
At all times keep your back straight, knees pointing in the same directions as your foot! Do not start with big weights, as back injuries can happen if the movement is not executed properly. Personally I don't do squats because my back just doesn't seem to like the movement. For an alternative move you should consider leg press. All in all, if you can do squats and have no problems with it, it is an excellent butt exercise to also conditioning your legs, gluteus and lower back.
What it targets
Squats target your Gluteus Maximus, your quadricepts and on the side a little bit on your lower back.
Tips for shaping butt
To put more pressure on your buttocks you can put your feet wider apart and turn your feet into a 45 degree angle regarding to your "centerline", if you are using dumbbells you can put the weights between your legs, as it's not very convenient to keep them on your sides. Another a little bit strange tip is to keep your toes up while executing the movement. Remember always when doing squats try to use as much as possible your glutes to deliver the power needed for the execution. Squats is easy to do at home, if you have free weights or something other heavy to carry in your hands you can use them.