Remove the top of any tin can. This will work with a 20-28 ounce can, up to a large coffee or juice can.
Punch holes about 1″ apart evenly in the bottom with a roofing nail, then punch a row of holes around the circumference about 3/4″ apart 1/3 of the way down from the top with the same nail but also enlarge the holes with a hole punch to about 3/16″.
Balance the can on bricks to allow air flow from below. Even safer, you could put some metal angles or pebbles in the bottom of a larger can and sit the burn can on them inside the larger can. (picture below)
Fill the bottom half of the can loosely with small organic material and burn it. (Chopped twigs or wood chips are about the right size, sawdust would be too dense and would not allow enough air flow.) The gap between the material and the top row of holes allows the volatile gases to build up and ignite. The top holes re-introduce oxygen and the top 1/3 acts as the after burner to burn off the gases. Keep an eye on it and pour water over it to douse the fire when the fire line reaches the bottom.
Wae Nelson’s gallon paint can kiln provides a more efficient burn and a more useful quantity, but this is really simple, and will make enough for a small potted plant as a test. Baby steps.
A larger can provides a windscreen, and is a safer way to contain the hot can.