To make an indoor greenhouse for tomatoes, estimate the desired size, making sure that the height is at minimum 2 feet taller than the tallest plant. Purchase the appropriate amount of metal shelving to accommodate the height and width of the greenhouse, and purchase metal framework for the top, bottom and braces to reinforce three of the sides. Wood is needed to construct the base.
Build a three-sided tower by connecting the metal pieces. Cut and waterproof a board for the wooden bottom of the greenhouse. It can be waterproofed by either covering the board with plastic or a plastic-like material and fastening down with staples; alternatively, sand and finish the wood with sealer. Lay the board across the bottom of the greenhouse.
Connect a single board or metal bar across the top to hang the lights on. Attach an inexpensive grow bulb to the board, and set up a timer. Center a sheet of plastic over the greenhouse so that all edges reach the floor. Fold the back pieces forward and the front pieces backward. Trim it so that the top covers the front opening by about three inches, and then secure it to the metal frame at the top.