Emily, I've tried negative reps, jumping pull ups and other exercises but I really made progress on my pull up strength when I started to use straps on a chin up bar.
Now I know most people will frown on using straps but I found they helped when I could focus on the actual movement without worrying about grip. (for the record, I weighed 105kg when I started trying to do pull ups and not all of it muscle dammit) Now able to do sets of 6-10... Wasnt easy took a long time but it worked for me
Once I was able to do a couple of pull ups without straps I would knock out as many reps as I could and then add straps for a couple more if possible.
Hope this helps