So I kind of want to add something to this and I hope it doesnt piss anyone off. This is what worked for me when i was 270+ lbs.
Obviously not having a ton of extra fat helps so adjust your diet accordingly.
Basically i couldnt get 1 pull up. I have tried bands and got REALLY awesome at doing band pullups... so I decided a different route.
Basically any workout where I was supposed to chins or pullups I subbed in Close Grip (the cable attachment where your hands are facing each other, people use it for low cable rows a lot) Pulldowns. 2 sets of 6-8 reps. Making sure to keep my shoulders and back set in perfect position and no swinging. Bring the handle down to the top of your chest and then fight the weight on the negative. I added 5lbs each time and didnt move up in weight till I got a full perfect two sets of 8
Ok so why you are doing that weekly (I did it on GSLP on days where weighted chins were programmed) You will also be doing some actual chinning. I found it easiest to get my first chin/pullup on the parallel or hammer grips handles. If you dont have them thats fine. But basically I would get to the top position of the chin then fight the negative from anywhere to 8-30 seconds for 3-5 negatives. I did the 5 set of negatives EVERY DAY. So go get one of those cool iron gym door frame dealios.
This got me from NO pull up at all. To my first pullup in about 2.5 weeks. Serious. Everyday do timed negatives. 3 sets of 8 seconds work to 5 sets of 8 seconds then start adding seconds to each set. By the time I was at 5 sets of 20 seconds I got an easy pullup.
With all due respect to T Bone if you tried the band thing and progressed and its not working. Try this.