As said before, a good fabricator takes all of his bits of knowledge from experience. Myself and a few of my buddies are starting to get into it. I am in M.E. major, and I have spent a lot of time observing what vehicles need in the field. What works, and what doesn't. Drawing on this, I get a few measurements, and spool up my imagination. I also get input from the person, since it is their bumper. What do they want? What don't they want? What kind of upgradability do they want? What is the vehicle used for? Is there a secondary purpose? (like on cherokees, anything to help stiffen the unibody) I take all of this and a piece of engineering paper and spew out a beginning concept. I then go back and on another piece of paper, make another drawing with a ruler, straight lines, proper angles, etc. Then autoCAD with dimensions. Then we get the materials and make it. A lot of times, a cardboard mockup is needed. The cardboard mockup insures that there is no fundamental problems, mainly, the concept of two objects occuping the same space (this is a problem). If there are any problems, it is very easy to cut/duct tape cardboard than it is to deal with steel. Finally, we build the little guy. Most of the stuff has been done with either a 220v stick welder, or more recently (due to the college location) a Hobart 135 flux core. The flux core barely gets the job done, but it makes it happen.
As one becomes more proficient, a lot of these steps can be skipped/reduced. But since I am new to this, I go the long way. Also it does keep my sketching skills up, since I took that class a year ago.