Measure the top of your table and cut a piece of plastic drop cloth that matches your measurements plus 2 inches. For example, a table with a 36-inch by 24-inch top will need a piece of plastic 38 inches by 26 inches. Lay the plastic on the table and wrap the excess down over the edges. Staple it in place.
Cut your 1-inch-thick dowel rods into four 12-inch lengths. Press one length against the edge of your table at a corner. Screw the dowel into the table, pushing the screw horizontally through the bottom of the dowel and into the edge of the table. Repeat with each of the other three dowels.
Fill your seed flats with soil and plant your seeds about ½-inch deep, spaced about 1 inch apart. Sprinkle soil over the seeds and lay the flats on your table. Water them until the soil is damp but not soggy. The plastic on the table should keep the wood from rotting.
Cut a piece of drop cloth about two times the size of your table top. In this example, the piece is 6 feet by 4 feet. Drape the plastic over the tops of your dowels, creating the roof and walls of your greenhouse. Stretch the plastic down to the edge of the table and staple it into place on three sides. Leave the fourth side open so you can reach the plants.
Place grow lights on either end of your mini greenhouse, adjusting the lamps so the bulbs are about 10 inches from the top of the plastic roof. Leave the lamps on for about 10 hours a day, turning them off at night. Your seedlings need rest, too.