Armored figures, such as the Stormtrooper and Luke/Coruscant, offer an interesting challenge for the customizer who likes figures with enhanced articulation. These techniques could probably be applied to making the ever-popular Boba Fett, although I haven't tried this yet.
This is a fairly advanced modeling project. You should be somewhat familiar with elementary casting and moldmaking. If not, take a few minutes to read about it. My other article (Moldmaking & Casting) at this site has more details as well as contact info for suppliers for some of the supplies you will need. You should be familiar with how to sculpt epoxy putty, since this is what we will use to fix casting flaws and limitations. Finally, you should be fearless with the Dremel mototool, since a lot of material will probably need to be removed from the GI-Joe figure to make the armor fit easily.
You will need the following materials and tools:
2-part Moldmaking putty, 2-part Casting resin, epoxy putty, panty-hose (I prefer black and so does my wife), a Kenner armored action figure, a GI-Joe type figure, contact cement, spray paint primer, Zap-a-Gap superglue, Dremel moto-tool, sandpaper, X-acto knife, scissors, acrylic paint and probably a few other obvious things.
Basically, you will be gluing castings of the Kenner armor to a pantyhose suit so that the armor "rides" over the figure. That way it will look good however it is posed, and the ugly articulation joints are hidden.
Here's how you do it: Make simple 1-part molds of all the armored sections of your Kenner action figure. Prepare your GI-Joe figure by grinding off inappropriate details. Contact cement pantyhose over the sections which the armor will not cover, but only at the edges. The material should be able to "move" over the figure, within a defined range. Thin-cast the armor pieces in resin, and before they're completely cured, fit them to your GI-Joe figure. When they're completely cured, take them off the GI-Joe figure and spray paint a primer coat where they will be painted with acrylic. Contact cement the armor to the pantyhose body suit. Epoxy putty and sculpt in the sections which were too difficult to cast (for me, the arm and leg sleeves). Sandpaper to finish and paint. Piece 'o cake, huh?
Well...not exactly. Here are a few extra tips about specific problems:
The leg and arm coverings are easier to cast in two sections. That way you just fit them over the arm and epoxy putty over the seams between the two sections.
For things like shoulder pads, you can use strips of thin vinyl to attach them to existing pieces of armor or the pantyhose. This gives them even more ability to flow with the pose. This seemed to be the only reasonable way to do the absurd pads on the Coruscant Luke figure. I've said this before: this costume looks neat but would have a hard time in the real world.
The Coruscant helmet is easy to cast in one piece, and works well. The POTF2 Stormtrooper's head will not work however, because it's too tiny (Unless you use a mini head or want a non-removeable helmet). The original Kenner Stormtrooper's helmet is bigger, but somewhat inaccurate. I sculpted my own, but you could probably use the POTF2 Luke Stormtrooper helmet. It's bigger and a much better sculpt.