Cut two pieces of lumber to the desired height for the ladder, usually between 5 and 7 feet. Use two-by-two or two-by-three boards for a sturdy ladder trellis, especially if you want to train heavy vines such as grapes (Vitis vinifera), which grow in U.S. Department of Agriculture plant hardiness zones 6 through 9.
Mark the sides of the boards for the rung placement about 8 to 12 inches apart. For example, for 8-inch rung placement, measure 8 inches from the bottom of the boards and make a mark. Mark the board every 8 inches up to the opposite ends of the boards.
Drill a pilot hole through the sides of the boards at each mark, using a power drill and a 1/8-inch drill bit that is slightly smaller in diameter than the 3-inch wood screws you'll use to assemble the ladder. The holes must go all the way through the boards.
Cut the rungs to the desired width for the ladder or about 12 inches. You'll need one ladder rung for every pilot hole along the sides. Use the same size lumber as you used for the ladder sides. Drill a centered pilot hole about 1-inch deep into each end of the ladder rung pieces.
Lay the side pieces on a flat work surface and line up the rung pieces between the longer boards. The narrow sides of the rungs should face out so the wide sides line up with the wide sides of the ladder side pieces.
Drive a 3-inch galvanized wood screw through the pilot holes in the side boards and into the pilot holes in the rungs to assemble the ladder.