Well first you have to find one you like or have an idea of what you want . Then you make it out of poster board or cardboard keeping in mind any shrinking, stretching or hammer forming you might want to do. Then you flatten out your pattern and cut it out of metal bend any sides or back sections that you may have setup. I use a small mechanical press brake but you can use a couple flat pieces of plate or angle clamped in a vice to get you through. I still do this allot to get tough to reach parts. Next find a flat surface and remember to try to keep it from twisting up as you weld and beat your seams together. I prefer to tack it together on the outside and weld it on the inside. The fillet weld on the inside allows you to grind a radius on any outside corners and helps to keep your lines straight.
I have a bit more shaping to do on the one above before I weld and metal finish it . It looks like its got some junk in the trunk.