The standard training method is to do one-armed lowerings, with control. However, that didn't work for me. Here's the method I used instead:
- Do about 100 chin-ups per day. This is hard without your own bar, so invest in one. Nowadays, they make lots of good models for doorways. Don't forget to stretch and make sure you recover properly.
- When you can do about 20 chin-ups in a row without rest, you're ready for the next level. Here's the trick: hold your chin-up bar with one arm, keeping your elbow at no more than 150 degrees. Take your other arm, and grab the bottom of the bicep of your chin-up arm (the part furthest from your shoulder). Use the supporting arm to give a bit of extra pull and stabilize your body while your doing these assisted chin-ups.
- Once you can do about 5 assisted chin-ups in a row, move your support arm a few centimeters closer to your shoulder. This reduces the amount of extra pull it can provide. Once you can do 5 repetitions at the new position, move your arm higher still.
- Once your support arm has reached your shoulder, it no longer provides any extra pull. And voilà, you're ready to do one armed chin-ups!
Good luck with your training!