The easiest way is to salvage a complete clutch from a car wreckyard.
If you only have a light engine, you can also do with a V belt that runs very loose around 2 pulleys.
Then use a 3rd pulley in the mid, between the two pulleys, that is pulled aside by a spring. This stretches the belt to engage drive.
You can make a clutch by attaching a pull cable to the arm that holds the 3rd spanner pulley, to pull it back to center (unstretch the belt) to release.
A V belt with no strain can slip over pulleys for a long time before wear spots will occur. It will give you plenty of time to change gears before the belt gets too hot on the spot where it rubs over the pulley. Mostly the belt will stop and the engine pulley will spin in the belt without rotating the actual belt.