Grab the chinup bar and jump up, so that you are now at the 'top' position.

From there, slowly lower yourself down. Work towards lowering yourself in 30 seconds. When are you able to lower yourself down in 30 seconds, you will be able to do a chinup.

Do 3-4 sets of lowering twice a week, and you should be able to do chinups real soon.

Yes it's not very easy, but it's effective. Lat pulldowns are not the most efficient way.