Wash the trays in 10-percent bleach solution. Rinse and let them dry.
Drill two rows of three holes down each side of the bottom of one of the trays. Use an electric drill equipped with a 1/2-inch drill bit.
Cut three 9-inch pieces from a 1-by-2 pine board using a saw. Place the wood pieces in a second plastic tray, one on each end and one in the middle of the tray.
Place the drilled tray into the tray with wood spacers so that the second tray is held off the bottom, creating a space for excess water to drain.
Fill the drilled tray with a 1-1/2 inch layer of well-mixed equal amounts of soil-less potting mix and peat moss.
Put 1 cup of hard red winter wheat seeds in a 1-quart mason jar. Add 2 cups of water. Put a piece of nylon netting over the jar top and hold it in place with a heavy rubber band. Soak the seeds overnight. Empty out the water and rinse the seeds through the nylon net under running water.
Invert the jar at a 45-degree angle in a dish drainer for another 12 hours. Inspect the seeds to make sure the white roots are beginning to emerge. Spread the seeds evenly on top of the growing medium, pressing them in place.
Water the tray so it is evenly moist but not soggy. Cover the wheatgrass tray with the third inverted plastic growing tray to block light.
Check the germinating seeds during the next four days, uncovering them just long enough to water and/or mist the seeds each day. Keep them evenly moist.
Remove the covering tray on the fourth day. Place the growing wheatgrass in bright indirect light. Water and/or mist it daily. Grow it for seven to 10 days, until the grass is about 8 inches tall.
Cut the grass close to the soil line with scissors or a sharp knife, and extract the juice immediately after harvest.