Sit down on flat bench or military bench and rest your elbows on the inner side of your upper thigh area (few inches away from your knees) OR on top of your legs (as depicted in the animation here). This start position will depend on your height and desired comfort level. Grab a hold of the bar no more than shoulder width apart. This is your starting position.
Lift the weight up until the bar is just a few inches away from your chin. Then, lower it back down but don't fully extend your arms. You should always keep tension in your biceps. Finally, repeat entire movement for the desired or prescribed number of repetitions.
Perform this exercise in a smooth and fluid motion
Don't extended your arms completely on the way down
SPECIAL NOTES & LEVEL of DIFFICULTY VARIATIONS
To increase the level of difficulty, slow down the movement or increase the weight.
This is a great alternative exercise to the dumbbell concentrated curls. The only thing you may find awkward about this exercise is that you may not find it perfectly comfortable at first. You may need to do a few practice reps to find the optimal positioning for your legs and elbows. Once you do, this exercise will seem as natural and comfortable as any other exercise.
Another thing worth pointing out is that this movement is all about doing the exercise in a smooth and "concentrated" motion. No sudden jerking motions and don't allow the weight stack to drop back down too fast. Thus, smooth, controlled and fluid is the way to go!