The skirts are curved to give it a clean look.  I don't have the tools to make this curvature so I rigged up the contraption on your right.  Basically, it is a  4" pipe about 46" long (or something close) clamped in a 6" vice.  The Skirting Panel (CF) was carefully inserted about an inch between the pipe and angle iron then clamped in place using several C-clamps.  Angle iron was used all along the long edges to better hold the piece in place for bending and to prevent kinking.  The portion of the Skirting Panel (CF) sticking up had more angle iron clamped at the end on both sides.  More clamps were used.  If you do not use angle iron on both sides along the entire edge, the metal will crimp when trying to bend it.  I used large crescent wrenches and a bit of muscle and pulled along the length of the pipe until the desired curvature was obtained.  A template was made for the correct shape so the piece would not have to be taken off to check for the correct curvature.   If you over bend it, just put it on a concrete floor and step on it to flatten it out a bit.

Once the skirting panel was curved, it was clamped onto the skirting frame.  Here you see how the more difficult bobtail end was done.  The black lines on the left are where slots were cut to aid in bending the panel.  A crescent wrench was used to twist the panel so they more easily overlapped each other.

At this point, the skirt panel was welded to the skirt frame using as many clamps as needed to insure no spaces were present between them.  A Dremel tool with a metal cutter was used to cut out any overlap.

When everything looked good, it was welded up.  A bit of grinding, filing, sanding and body filler did the trick.

A paper template was made for the truck cutouts.  It was glued on the skirting and an outline made.  The piece was then cut out, filed, sanded, etc.  Once you did the skirting for the bobtail end, the other ones are easier (6 total) since they don't require a curved end.