Step 1: Selecting Lumber and Getting Tools
Purchase a bundle of lumber. I choose to buy 48"x1.5"x0.25"pine due to the low cost. Select 32 pieces that are straight and lay flat. I used a small diameter drill bit for predrilling holes, and 0.5" length screws #2 diameter for fastening the wood.
Step 2: Forming the Criss-Cross Pattern
Using a square, or a block of wood with square sides, align the ends of two pieces of wood and fasten them together orthogonal one to the other. Then, moving down the piece of wood on the bottom with gap spacing of 3.5" between the pieces of wood, attach 9 pieces of wood so that there are 9 total. You will need to align one side of your trellis against a wall or something straight to keep the wood pieces at the correct angles.
Note, my block of wood had a width of 3.5".
Step 3: Flipping Over the Trellis and Attaching Complementary Pieces
Grab hold of the main piece of wood that has 9 other pieces attached to it. Carefully turn over your trellis. If some of the pieces rotate you will need to adjust them back to the correct right angles. Attach the 8 pieces of wood needed so that both sides of the trellis have 9 complementary pieces. Keep the gapping at 3.5". You will know if you are doing this correctly if you are producing 3.5" square shapes in the trellis design.
Step 4: Filling In the Gaps on the Ends
To make you trellis long enough you need to add 3 pieces of wood to each end. Then you will need to flip over the trellis and add 3 pieces of wood to each end. That is a total of 12 pieces of wood. As you are attaching the wood mark the existing corners of the trellis so that you know where to chop the wood to make a straight side. This is also a good time to add strength to the main body of the trellis by inserting additional screws in the middle.