1. Nope, you don't have to buy a whole meter, or yard, of fabric. Most stores will let you buy partial yards (or partial meters). It depends on the store; the stores around me will sell 1/8 yard as the smallest increment. They will also sell 1/2, 1/3, 3/4, 2/3, or 1/3. You will, however, be forced to buy the entire width of the bolt, usually 44 inches or 60 inches, depending. Some stores, especially online vendors, however, have strict 1-yard or 1/2 yard minimum orders, in which case, you have no choice but to buy that amount. As for the triangles (I assume this is what you mean by 'diamonds'), my preferred method for that sort of thing is Heat-n'Bond followed up by machine overcast applique. You could paint them on, as well.
2. Your reference photo doesn't really shows this in any sort of detail, so I cannot provide very much advice, however, it is simple to cut perfect circles - trace a plate. If it needs to be bigger or smaller than the size of the plate, then measure a few inches from the original line you drew.
3. I assume you mean the yellow scarf under the sailor collar? Measure around the neckline of the shirt, under the collar, and add about 10 inches to that measurement to account for material used tying the knot. This will be the diagonal length of the scarf. If the scarf is to be a regular square, a simple bit of math will give you the length of the two sides. If it's too big, just trim it down. Of course, if it's too small, there's not much you can do about it, unfortunately. Make a practice one, if you can, until you are sure what size you want.
4. I'd advise cutting a stencil and painting it on, or printing one out on photo-transfer iron-on paper. You can get this sort of thing at most craft stores, and sometimes at office-supply/printing supply stores. Incidentally, if you are looking for a pattern to print out or trace onto a stencil, look for a G-clef, or treble clef (not a music note!).
5. That is a basic D-ring belt. Go though both D rings, fold the belt back over the second D-ring, and go back through the first. It can then be easily adjusted/cinched for tighter or looser fit.
Pictures and tutorials from my costuming portfolio at Syagria's Cosplay Site!
Star Trek Original Series Uniform!
Original Creation for AB13
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PAX East, Anime Boston 2013