Essentially two pieces of wood joined together with a hinge. At the other end they're held apart using a threaded rod, and then I use a motor to drive a nut on the rod so the boards very slowly close together at the same rate as (but opposite direction to) the earth's rotation (roughly 15°/hour). My camera is mounted to a ball head on the top board. This means, relative to the camera, the stars are stationary so I can do long exposures without getting star trails.

There are commercial star trackers like Move Shoot Move that work in the same way, but the DIY method is much cheaper and more fun 😁