@greg67 There's a weighted pull up workout program which can help. Every workout you increase the number of reps, until 5 and then you drop the reps and increase the sets for the next workout. So 9 training sessions would look like this: (sets X reps)
3x3, 3x4, 3x5, 4x3, 4x4, 4x5, 5x3, 5x4...