Lie down on the floor and rest your heels (whole foot if you can) up against the frame of the weight stack. With your arms down, grab the bar with your palms facing up and at shoulder-width apart. This is your starting position.
Curl/lift the weight as far up as you can (as far as your natural arm movement allows). Then, lower the weight back down to the original starting position and repeat entire movement for the desired or prescribed number of repetitions.
Perform this exercise in a smooth and fluid motion
Don't push off with your legs too much because this is cheating and does nothing for your biceps
SPECIAL NOTES & LEVEL of DIFFICULTY VARIATIONS
To increase the level of difficulty, slow down the movement or increase the weight.
This is one exercise that you won't see people do that often at the gym. However, it is very effective for the biceps and offers a very good extension/stretch for the lower head of the bicep muscle.
It's a very good exercise to throw into the mix once your bicep workout routine becomes stale and you find yourself hitting a developmental plateau.
One key element to point out here is to make sure your head is completely resting on the floor. There is no need to raise your head up and look at the weight stack while executing this exercise. Lifting your head up will only cause your neck muscles unnecessary strain and possibly even discomfort.