In one sentence, describe what the following article is about:

with your left hand by gripping the screw-joint. The screw-joint is located underneath the bell. Grip it firmly with your left hand—you’ll be able to support the trombone by using this hold. Now, make a gun with your left hand, extend your index finger so it touches the mouthpiece, and wrap your thumb around the bar closest to it.  Use your remaining 3 fingers to grip the vertical bar below your index finger. Make sure the slide is locked when you aren't playing the horn. If you leave it open, the slide can fall off and get damaged. The slide lock is typically located at the region where the main slide connects to the bell section. Use the tips of your index and middle fingers and your thumb to grip the main slide. This is the hand you use to move the main slide forward and backward. Again, make sure the slide is locked—one small dent in the slide can cause it to become stuck and unmovable!  Relax your right-hand grip and shoulders. Always maintain a firm grip on the trombone with your left hand, even as you relax your right hand. This loosens up your chest and stomach, which allows your diaphragm to fill up with more air and helps you inhale faster. If you're sitting, sit as far back in the chair as possible and always keep your back against the back of the chair. Sitting on the edge of the seat restricts the air flow from your lungs.  Avoid sitting whenever possible. If you sit, place your feet firmly onto the ground while keeping your body centered.

Summary:
Hold the trombone Grip the movable part of the slide gently with your right hand. Keep your body centered when playing.