Guarantee will work - for either your first - or getting your "total" higher. Here's the theory then I'll work back to zero reps now
you do 4 "sets" - max on each with a full 5 minute rest (you aren't doing a work out so you want full rest for effort - think a sprinter workout)
3 days a week
each workout you do 1 more rep (or more) of the 4 set total then the prior workout for example if you do 3/3/2/0= 8 reps - next time you do 9 reps. If you can't get 9 you do a 5th set.
full extention on bottom - no bounce
Now when at zero - you must pull as hard as you can for as long as you can- THAT is what is going to make you stronger - its the miss, not the makes that cause the stress / recovery / stronger cycle - (note if you can do pullups - the last set "miss" should be as long and hard as you can put also)
You will get that first one faster then "assisted" reps, then you will quickly start having success -
My experience when I was late 50's - I was doing crossfit regularly but pull ups were aweful - the 8 rep example above was me - in 6 weeks worked up to 13/13/12/11=49 - my son was an 8th grader - needed to pass a fitness test to run on HS XC team - zero reps - 5 weeks hit 8 reps
It will work much better then you can imagine - don't talk yourself out of the 5 min rest between sets - key. GL