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.
Lie flat on a bench, with your arms straight over your shoulders, holding dumbbells.
Bend your elbows until your arms are bent at approximately 90 degrees.
Extend your elbows back to the start position and repeat.
Moving elbows around.
Using excessive momentum.
Rushing repetitions.