Step 3: Shape your materials
The frame is 4 pieces: the main arms, which support most of the weight and are attached to the back axle on either side, the U brace, which ties the arms together, and the spine brace, which ties all of that to the frame. The measurements of these will change based on what your bike's tire size is and what you're rack's width will be.
Note: I suggest drilling all holes in these parts before cutting them, as doing so after is more difficult.
You can bend the pieces out on any hard surface, though I find a vice to be best since it holds the piece steady and lets you get nice right angles and corners rather than more rounded ones. A picnic table/workbench and a C clamp is a good alternative as well. Make sure to bend the main arms at the same time so you don't wind up with one that's more heavily angled than the other. Compare them back and forth frequently if you can't do them both at once since having to bend one back too far will fatigue and weaken the metal.