Grab a hold of the crossbar with your palms facing towards you at shoulder-width apart and let your body hang. Your legs should be bent at your knees and crossed at your ankles. This is your starting position.
While keeping your elbows tucked in as much as you can, raise yourself up until your chin passes the bar. Then, lower yourself back down but do not completely extended your arms (always keep them slightly bent at the bottom of the movement). You should always keep tension in your biceps. Repeat entire movement for the desired or prescribed number of repetitions.
Perform this exercise in a smooth and fluid motion
Keep your elbows tucked in as much as possible
Don't extended your arms completely on the downward portion of the movement
SPECIAL NOTES & LEVEL of DIFFICULTY VARIATIONS
To increase the level of difficulty, slow down the movement or use a weight belt with chain that allows you to hang/load a weight plate between your legs.
This is not an easy exercise to execute. In other words, it's not for absolute beginners because it requires some pre-conditioning in your arms and back. Thus, if you're a beginner, you should do other bicep exercises.
For those who are able to execute this movement properly, bicep mass gains are a given because this exercise is one of the best (yet rarely used) for building slabs of muscle onto the biceps.
One note worth mentioning is that if you find this exercise hard to do, then do partial reps. This means, raise yourself up halfway if not all the way. Even partial repetitions are effective.