Stand straight holding the bar up at shoulder height.
Press the bar straight up until your elbows are almost fully extended.
Slowly lower down until you're at a 90 degree bend in the elbows and repeat.
Elbows being behind your hips.
Arching your back too much.
Using legs for excessive momentum.
With the bar racked at shoulder height, grip the bar at just wider than shoulder width with an overhand grip. Lift up the bar and take a couple of steps back. Stand straight, with your feet shoulder width apart.
Start by pressing the bar straight upwards, squeezing through the shoulders until your elbows are almost fully extended. Then slowly lower down until you're back at approximately a 90 degree bend. Move your head slightly back on the way up to ensure you can push up in a straight motion. Keep your forearms, wrists and elbow aligned at all times and brace your core throughout to keep your back straight. Repeat for the desired amount of repetitions.
Avoid losing alignment between your elbow, forearm and wrists when pressing.