Barbell Hip Thrust
1. Begin seated on the ground with a bench behind you. Roll a loaded bar onto your lap so that it is above your hips
2. Settle your shoulder blades against the bench and lean back against it.
3. Drive your feet into the ground and push through your glutes to extend your hips upwards,keeping your chin tucked to prevent your back from over-arching.
4. Lifting the bar until your body is parallel with the ground. Squeeze your glutes at the top.
5. Lower your hips until you are close to the ground then repeat the motion
I recommend getting a barbell pad to make this more comfortable, maximize the weight you can lift, and prevent bruising. This is the one I use.
Dumbbell Alternative: dumbbell hip thrusts