Mark out a garden bed measuring 4 by 6 feet, to match the dimensions of a typical panel of cement-reinforcing mesh or hog wire. Dig a 12-inch hole at each corner and at the midpoint of each long side with a post-hole digger. Insert a 4-foot-long, 4-inch by 4-inch pressure-treated post in each hole.
Mark the posts at heights of 18 and 32 inches above the ground. Tie or nail a support -- it can be bamboo stakes, PVC piping or a pressure-treated 2-by-4 -- across each pair of posts at each mark, to serve as supports for the horizontal cage material.
Lay a mesh panel across the supports at the 18-inch height and a second panel at the 32-inch height. These will support the tomatoes at the heights where they typically produce the largest clusters of fruit. Tie the panels in place using rebar tie wire.
Plant the tomatoes underneath the horizontal tomato frame. Gently train them as they grow. Pull the central stem through the 6-inch-square openings in the mesh.