Mirror balls -- also called disco balls -- are covered with an array of tiny mirrors that reflect a pattern of small rings of light when lit correctly. The balls can range in size from tiny keyring models that are intended as novelty items to giant spheres that are a metre (several feet) in diameter. Commonly used mirror balls for performances, nightclubs and parties are generally 20 to 40 cm (8 to 16 inches) in diameter and may be full or half spheres. They are generally suspended from a motorised unit on the ceiling, which allows them to revolve at various speeds, spinning the light rings across the walls and floor.
Suspend your mirror ball where you want it to hang, making sure it is firmly attached, and poses no danger to anyone below it.
Select two matching lights for your mirror ball. If you have access to theatrical lighting instrument, ellipsoidal reflector spot units with adjustable irises offer the most control. Small, tightly focused lighting units known as pin spots are also common for mirror ball lighting. For small at-home installations, clip-style reading lights with adjustable necks may work.
Position one light on each side of the mirror ball, slightly below the midline (widest portion) on a sphere or the rim on a half-sphere -- about 15 degrees below the ball is a good angle for most situations. Aim the light at the mirror ball and either adjust its features so it lights only the mirror ball and not the surrounding areas, or adjust the lights' distance from the mirror ball to produce the same effect.
Clamp or otherwise affix your lights in place once they are focused on the ball. If placed correctly, each one will light exactly one half of the sphere, reflecting light to all sides and below the mirror ball. For half-sphere mirror balls, you may need to position your lights lower to avoid excess light spilling over the top of the bulb.
Most stores that sell mirror balls also sell pin spot fixtures, which generally have higher power and more control than a clip light yet are more affordable than ellipsoidal instruments. If you want multiple colours of light rings, use theatrical colour filters on the lights and use more lights from different angles -- such as three lights at 0, 120 and 240 degrees around the circumference of the sphere at the standard 15-degree angle -- to create more areas of colour.