Put your feet on a box or something, so that you're hanging from the chin-up bar in a posture vaguely similar to how you'd sit in a chair. Then use the legs as much as you need to, and as little as you can get away with, to perform six to eight pullups.
That's one set. Gradually add additional sets (with very good rests after each) until you reach a total of 20-30 reps for the whole workout. At that point you should be able to perform a strict, dead-hang pullup with no problem.
OR:
Go with negatives. Jump up to the top position, then slowly lower yourself, using your lats to resist gravity, to a dead-hang position. Jump back up and repeat. Six to ten of these every other day should leave you sore as hell after the first couple workouts, but then you'll be okay.