There are many ways to build a trellis. You can create an ornate trellis using 1-by-1-inch lumber or even create a frame with wires to support your plants. However, you can make a very simple trellis by using pre-made lattice and 2-by-4-inch lumber as supports. Mount this type of trellis on a wall, to support climbing fines and roses, or extend the supports downward as posts in the ground to support the trellis.
Measure the 2-by-4-inch sidepieces for your trellis, using the measuring tape. If you're mounting the trellis on an exterior wall, measure the sidepieces to the exact height of the trellis. If you're using posts, measure the sidepieces 3 feet longer than the height of your lattice. For example, if you're making a 6-foot trellis, cut your 2-by-4 lumber to 9 feet long, using a saw. If the lumber is only 8 feet long, allow the top foot of the lattice to extend above the posts and add a 1-foot piece above the posts.
Place the cut posts on the ground with the lattice on top. Drill through the lattice with a 1/16-inch drill bit and secure the lattice to the posts with screws. The holes are pilot holes to help keep the lattice from splitting.
Measure the distance between the support posts and cut two 2-by-4s to fit between the posts to give the lattice horizontal support. This is especially important with wide trellises.
Drill pilot holes and screw the horizontal supports to the lattice, using the drill.
Drill through the side supports and use long screws to secure the horizontal supports to the side supports.
Dig two 3-foot-deep holes for the posts, using a shovel, if you have made a self-supported trellis. Place the posts in the holes and backfill. Pack the soil as much as possible.
Drill holes through the horizontal and vertical supports if you are screwing the trellis to an exterior wall. Secure the trellis to the wall with long screw and a drill. Place screws around the perimeter of your trellis, spaced 6 to 12 inches apart.