Hold the lyre with the clamps facing toward you and the screws facing away from you. Untwist the screws so that the clamps are all the way open.
Secure your trombone so it won't fall while you're attaching the lyre. The bell should be facing toward the ground.
Slide the clamps over the rim of the bell on the right side, next to the slide. Adjust the lyre so it is roughly an inch away from the slide, enough to allow free movement of the slide without hitting the lyre.
Twist the screws until the clamps are firmly attached to the bell. They should be tight enough that you can't move the lyre at all by wiggling it around. Grasp the lyre tightly with your hand and test it to make sure it's tight enough.