Glute thrusters with the bar locked in.
Tuck your chin and finish with hips fully extended, not over-arched.
The hip thrust already isolates the glutes better than almost anything, and the Smith machine removes its one annoyance, the wobbling bar. With your upper back on the bench and the bar tracking a fixed line over your hips, every ounce of effort goes into driving to lockout, where the glutes are at their strongest and squeezed hardest. The hamstrings assist from below, and the guided path makes heavy loading feel far more secure than a free bar rolling on your lap.
It is more stable, faster to set up, and easier to push hard because the bar cannot roll or tip. The barbell version demands more control for the same stimulus. If your gym's Smith machine lines up with a bench, it is a genuinely great thrust station.
Your shoulder blades should rest on the pad with your torso able to hinge over the bench edge, most standard flat benches work. At the top of the rep your body should form a straight line from knees to shoulders, if you cannot reach that, the bench is too tall.
Positioned so your shins are vertical when the hips are locked out. Feet too close sends the work to your quads, too far turns it into a hamstring drag. Vertical shins keep the glutes in charge.
