Choose an appropriate weight matching your fitness level. Lie down on a flat bench and grab the barbell using a pronated shoulder grip width (radial part of your hand facing you). Hold the bar straight over your chest.
Lower the weight slowly in an arc behind your head by flexing your elbows. Keep your elbows pointing forward (not sideways) all the way through. Continue with the motion (hands behind your head) until your elbows are in line with your head and shoulders. Breathe in as you go down and breathe out as you go back to the starting position. Repeat as required.
Make sure your starting hand position is correct. Be careful about moving your elbows sideways.
Be careful about wrist dorsiflexion while shifting the weight.
Keep your torso upright with no lower-back arching (lower-back lordosis), especially on the way down.
Choose your resistance to match your abilities and fitness level. Start with lighter resistance to master the technique well.