Sit down in a seat thats positioned so the path of the bar is just in front of your face. Grab onto the bar at just wider than shoulder width.
Start by pressing up to unlock, then lower the bar until its approximately at your chin. Then press straight upwards, squeezing through the shoulders until your elbows are almost fully extended. Ensure that your elbows stay in line with the front of the hips when going up and down. Brace your core throughout to keep your back straight. Repeat for the desired amount of repetitions then at the top of the final rep lock the bar.
Avoid having the elbows come too far forward or back from the hips.