do assisted chins until you can do a good set. If you have access to a way to do chins for most of the day, you can try doing half of what you can do for a set, but a few times a day everyday. For instance, work your way up to 5-6 chins. Then do sets of 3 chins, but 3-5 times a day every day. Ypu should be able to add 1 rep a week for a few weeks, then take a week or 2 break from it and do it again. I did this on and off a lot when younger, and I got to the point where I could do tons of bodyweight pullups or chinups. Best way to "jumpstart" bodyweight movements like pullsups and dips IMO.