Start with the barbell on your shoulders, below your chin.
While holding the barbell on your shoulders, lower yourself into a squat.
From the squat push yous self to a full extension, with the barbell held over head. Keep your elbows straight. Then come back to your first pose and repeat again.
Your heels should be firmly planted on the ground except when going to a full extension.
Don't put your weight on your toes except when going to a full extension.
Don't bend your elbows with the barbell held over head.