For each shelf you need a plastic container that fits under the bed. I believe it's about 40 inches long. You also need a 4 x 8 one inch styrofoam board and some of the small plastic bathroom cups.
Using a 2" hole drill, cut holes about 4" apart. Cut two pieces and glue them together. Fill the plastic container with water mixed with liquid fertilizer. Put in about three gallons and mix well.
Place the styrofoam on top and the lettuce pots fit great! The roots will grow in the nutrient reservoir.
I start my lettuce in small 1 1/2" paper pots that I make. They will fit into the bathroom cups. I punch holes in the bottom and sides of the bathroom cups so the roots can grow out and into the water!
(What you see in the picture are some greenhouse hydroponic net pots, but I used bathroom cups for several for a couple of years.)
Happy growing!