1) Make your initial leg armor pattern assuming that the end of the leg armor will sit high up on the person who's wearing it. Use the length of this pattern to determine the size of the squares you'll need. Order said squares before you discuss the matter further with an armor-savvy friend, who tells you that the armor will actually sit around hip level.
2) Knowing very little about the various properties of metal, decide that it would be really rad to use stainless steel instead of aluminum because you like the stainless finish better and you figure that because you're using 24-gauge, it will be easy to cut with snips that are rated for up to 18-gauge steel. Don't realize that they meant 18-gauge MILD steel, which is much easier to cut.
3) Order the metal in the form of strips so you only have to make a single cut for each square. (This is actually a Donny Do idea because it makes your life much easier without adding a crapload of custom cutting cost to your order) Discover that 24-gauge stainless is a royal biznatch to cut without power tools. Make about 25 squares (out of 144), severely tiring out your forearms. Do research. Order an incredibly kickass Milwaukee Sawzall. (This actually wasn't a bad idea, either, because the one I got is top of the line and I bought it new from Amazon at over 50% off. Considering that a reciprocating saw is a staple power tool and we didn't have one already, I'm not going to whine.)
4) Put your pattern on the wearer of the costume. This time, place it where it will actually sit. Realize that it's way too long and that your squares must therefore be 1/2" shorter. Swear. Decide that rather than spend hours cutting the stainless out and then shaving down all those squares, you'll order enough aluminum to redo the squares and hope that you'll find some future use for a few yards of 2" stainless strips.
Feh...at least I'm making the surcoat the Donny Do way. I cut out the pattern from my cheapass lining first and had Rando try it on before I even thought about messing with the good brocade. I discovered that it was too long, the front opening was too narrow, and the arm openings needed to be cut in more (they bind up a bit under his arms and when made of brocade, they'd add too much bulk under the jacket worn over the surcoat). I have to go get a French curve so I can redraw the armscyes perfectly smoothly (for now, I just traced them roughly using a tank top as a guideline to see what the curve looks like and then had him try on the garment to see where the markings fall, and it looked OK to me) and then recut the lining and verify that the final result is good. If so, then we proceed. w00t. Based on the finalized parts, I can proceed to make the collar and trim, so at least that's good. However, I think I might have cut the garment a wee bit short (I was dumb and didn't realize that when he wears a belt over it, that will hike it up somewhat, and I measured the new bottom edge sans belt...blergh), so I may have to redo the lining completely once the pattern is final. Like I said, it was cheap, so I really don't care. It'll be better to recut it than to add a silly little strip at the bottom of each half.