Warming up will get your blood flowing and help prevent injury. To warm up, do some cardio exercises like walking or jogging. You should also do some active stretching, like arm circles or arm swings. If pull-ups are hard for you, it may feel natural to "scrunch up" your body while performing them. This can put extra stress on your neck and back muscles, making the entire workout process harder. To avoid this, set your shoulders by pushing them back and keep your spine arched. For those who don't know any better, pull-ups may seem like the ultimate test of arm strength. However, your back muscles are just as important, if not more so. To make your workout easier and more effective, try to use the muscles on your back and around your armpits to help pull yourself up. Specifically, try to engage your latissimus dorsi muscles (“lats”) and rear deltoids (“delts”). When performing your pull-ups, try crossing your legs close to the ankles. Though it may seem unimportant, this can reduce some of the pressure in your arms and will make it easier to maintain proper form while exercising. When crossing your legs, you can either bend your knees or keep them straight. Neither will affect the overall routine, so choose whichever option feels best. Assisted pull-ups will allow you to perform the same basic motions you would for an ordinary pull up, but with a little extra help so you can more easily complete the exercise. Since these closely mimic actual pull-ups, they're great for getting your form right before you attempt the real thing. You can do assisted pull ups by:  Using a pull-up machine. Looping a resistance band around the bar and your foot to support some of your weight. Standing on a stool with one foot. Asking a partner to hold your feet or legs while you workout.

Summary:
Warm up for 5-10 minutes before you start. Set your shoulders and arch your back. Use the muscles in your back, not just your arms. Cross your legs while working out. Perform assisted pull-ups if you’re struggling.