Build your arm and grip strength by holding on and hanging. This is also great for spinal decompression.
Jump up so that your chin is above the bar or handles, and stay at the top position.
This is all about the negative movement - jump up and slowly lower yourself down.
Using an assist pad on the machine or a band attachment, practice the full movement.
After practicing the previous exercises for a while, you'll get strong enough to do a chin up all on your own!