Block pulls

Set up a barbell on blocks or mats so the plates are elevated (typically to mid-shin or just below the knee), standing with your feet hip-width apart.
Hinge at your hips, bend your knees to grab the bar, pull the slack out of the barbell, and brace your core tightly with a flat back.
Drive through your mid-foot and heels while extending your hips and knees together to stand tall, squeezing your glutes at the top.
Hinge at your hips to lower the bar back onto the blocks under control, letting it come to a complete stop before the next rep.

Tips

Before initiating the pull, take the slack out of the bar by applying gradual upward tension until you feel the plates lightly load against the blocks, then drive through your feet to begin the lift.
Set your hips at a height that places your shins close to vertical and your shoulders directly above or just in front of the bar before you pull.
Lower the bar back to the blocks by hinging at the hips first, then letting the knees bend as the bar passes them, and allow it to come to a complete stop before the next rep.

Jerking the bar off the blocks instead of building tension first causes the hips to shoot up before the shoulders move, breaking the starting position.
Allowing the lower back to round as the bar leaves the blocks, rather than maintaining the neutral spine established at setup.
Resetting immediately after touching the blocks without letting the bar come to a complete stop, which turns the set into a continuous bounce rather than distinct dead-stop repetitions.