What's the best way to work up to a one armed chin up?
In order to build the strength to perform one arm chinups you have to focus your efforts towards a strength based progression. That means instead of performing high rep unweighted chinups; start doing them weighted.
If you progressively build up the strength to perform a 1X bodyweight weighted chinup; a one arm pullup becomes more possible. That means you have to work within the parameter of 3–5 reps, progressively adding weight to your chins every week.
And although that approach to achieving the one arm chinup is practical; you can't count on it entirely. You need to incorporate supplemental exercises that aid in skill development and reinforcing the neural patterns associated with 1 arm chins. Below I'll list some of the supplemental movements that can aid you towards performing 1 arm chins.