One of my recent DIY projects was to build a pea trellis. Here is how I did it.
The trellis consists of two 6' x 4' rectangles, with a cross brace in the middle of the 6' sides. These were fastened together with 3" decking screws. The lumber used here are 2" x 4" untreated studs. Although I might not have the carpentry skills to make perfect 90 degree angles, these frames are very strong. To give the trellis some protection against the elements, I sealed the wood in boiled linseed oil.
For a growing support, I tacked 1" UV-resistant black plastic netting to the frames. I tried to make the netting as a tight as possible so that there wouldn't be too much sagging under he weight of the vegetables.
Finally, I wanted to make this trellis collapsible for storage during the winter so I added to brass door hinges at the top, which I salvaged from one of my other remodeling jobs around the house.
The trellis frame just sits on top of the frames of my existing garden beds. Although the weight alone makes the trellis quite stable, I decided to bend a few nails around the base of each side to lock things in place a bit better.
To train the pea plants onto my trellis, I took two approaches. At the base of each frame, I try to weave the young plants into the netting to get them started up the sides. For plants in the middle, I hung twine from the top of the hinge region. To weight the twine, I tied some old cabinet knobs to the ends of each string. Then, I just try to wrap the plants around the twine. They don't stay very well at first, but after a few days of training, they hold on pretty well.