Forget straight lines for a moment and think outside the ubiquitous square box. Rebar is flexible and can easily be bent to make arches, tunnels, domes, and free-form curvy shapes. There is really only one step in making a rebar trellis; putting the rebar where you want it and tying it in place with wire. There are several design considerations however. This instructable will cover the simple "how-to" and the more complex "why" of design considerations.
A rebar trellis can also be converted into a cement structure by covering it with a mesh material and then plastering it with cement. My home was made using a trellis-like support for nylon fishnet and cement. A second-generation trellis now covers my home for shade. It could also be cemented someday, allowing for a third-generation trellis over that, and so on. The end result would evolve into something resembling a termite nest-like city, and create a radically different way of life for everybody involved.
Step 1: Tools and Materials
I use mostly 1/4 inch rebar, but 3/8 inch rebar is also useful, especially at first to establish the major lines. Bending the rebar by hand is easy. Pliers are needed only for sharp bends. 1/16 inch galvanized iron tie wire is used for tying it all together.
When you tie the rebar together at the intersections, you have the option of either twisting the wires clockwise or counter-clockwise. One way is usually better than the other, depending on how the rods overlap each other. If the wire doesn't stay tight, the joint will be loose. The direction you turn is something you have to decide on each time you twist, and it is something I have a hard time explaining to people even in person. Good luck learning it on your own! Practice and keep thinking as you go.
One hint: pull as you twist. On the last twist, bend it all over if you can, to avoid snagging on the wire ends later.