Step 5: Make the Spine of the Bracket
Pictures are better than words here, but here's an approximation. This is the part that will vary considerably with the design of your car.
Bend the two wire legs so that they curve down, meet, and turn around. Then bend the remaining legs outwards, like arms outstretched to hug the antenna post (which they will). Test fit it, and bend it around as needed until you like the look and the cap is more or less centered over the hole. Wire's pretty forgiving.
If you're using steel wire, feel free to solder up the spine. This will make the bracket stronger, but also decrease your margin for error, so make sure it's right where you want it before you plug in the iron.