A mason jar is one of the simplest ways to start the Kratky method. You need a container, a lid with a hole for a net cup, a growing medium, and nutrient solution.
Cover the jar
Clear glass lets light reach the solution, which encourages algae. Wrap the jar in dark material or paint it, leaving the top open for the plant.
Fill and plant
Fill the jar with nutrient solution to just below the bottom of the net cup. Put a seedling in the net cup with a medium such as clay pebbles, then set it in the lid.
Watch the air gap
As the plant drinks, the solution level drops and leaves an air gap. Roots grow down into the solution and into the air above it. Do not top up often, because the air gap is part of how the method works.