Two methods that will work: “grease the groove” and ladders.
Grease the groove = do many sets every day, well short of failure. If you can do 8 at the moment, do sets of 4-5. After a month, check your max and adjust number of reps accordingly.
Grease the groove, IMO, is the way to go. My brother got pullup bar for his 10th birthday, and since then every time I went downstairs I did 10-15-20 pullups, and I still do them to this day , 7 years later, every time I go downstairs. Grease that groove!
Also, I’m into heavy metal and I went to ozzfest a few years ago and got a free Marines t-shirt for doing 20 pullups, so if that’s not incentive, I don’t know what is.