The stronger your grip is the easier pull ups will feel. I suggest getting a Captains of Crush Gripper and using it a few times per week.
Breathe slowly and controlled.
Fasten a weight belt around your waist or ankles. Hook a dumbbell or weightlifting plate to your belt. If you're new to weighted pull ups, add somewhere between 5 to 10 pounds. If you're an advanced athlete, go for 20 or more.
Grasp the bar using your preferred grip. Bend your knees so that your feet aren't touching the ground. Allow your elbows to straighten out so you're in a fully-extended hanging position.
Pull yourself up until your chin clears the bar. Lower yourself slowly back into your starting position. Remember not to rest your feet against the ground during any phase of the weighted pull up.
Finish up your set with the desired number of reps. Give your muscles a few minutes to rest, then repeat the exercises.
Don't rest your feet on the ground during any phase of the weighted pull up.
Don't attempt if you have any shoulder problems.
Don't cut the motion short. Make sure you go all the way down, and all the way up when you do a pullup.