If you're looking for a way to make pull ups even more challenging, the next step would be to add weight to the movement. You must be able to do perfect reps of a bodyweight pull up before trying to add weight.
You have many options for weighting a pull up. Wearing a weight vest is an option. You can also wear a dip belt and attach a weight by using the included chain. If neither of those are available to you, you can hold a dumbbell between your feet, but you may need some assistance getting it into position once you are on the pull up bar.
Once you've decided how you will add weight to the pull up, follow the same mechanics for a bodyweight pull up. Avoid overarching in the back and complete the movement to full range of motion. Make sure you start with your arms fully extended and you get your chin over the bar at the top of the rep.
After you have completed your reps, safely remove the weight from your body.