This is actually fairly easy - if you have four key things:
Here's the technique for getting an Italian Ren, cotehardie, or other closefitting bodice that fits!
First, decide if you want it to be a four-piece bodice (seams up front, back, sides) a five piece bodice (the kind with the panel in the front with embroidery/beading on it, looks like a dickey or an Elizabethan partlet, sort of), or a more-than-five-piece, as with some of the particolored cotehardies. I'm going to describe the technique first for the four-seamer; once you've got the concept it's easy enough to adapt to five or more.
Cut two lengths of the cheap fabric - about a foot longer top-to-bottom than you are from shoulders to waist (measuring over the boobage), and about a foot and a half or two feet wider than you are from shoulder to shoulder (also over the boobage). Pin the shoulders together as close to the outside edge as you can, about an inch from the top, pins horizontal to the ground. Put this 'neckhole' of sorts over your head. Pinch about six inches of the excess together in the center of front and back. Pin together along the sides and along the slope of the shoulders, loosely fitting it to you. Now adjust until it fits well - being careful to hoist your boobs and then pin to support them (usually lifting and compressing towards the center works best). Alternate adjusting the sides with adjusting the front, evening it out as you go. Try to keep the grain of the fabric perpendicular to the ground, as that makes it easier when you go to cut out the 'fashion fabric' (I love that phrase! :-). Also try to keep your seams straight, and even between one side and the other (i.e. not with one side-seam coming out in front of the arm and the other behind ...)
This will take between fifteen minutes and an hour; it gets easier with practice. Take as much time as you need here, for this is what makes the fit right. Please DO play with the fabric, get a feel for how it 'wants' to lay. The key is to keep messing with it until it looks and feels 'right', without puckers or strains or other strange occurrences.
Now take your marking implement and draw along where the pins are, making your marks clear. These will be your seam lines. Then go from the seams down the neckline and around the arms, marking where the edges will be in the finished piece. The fabric and the body will tell you, after a fashion, where the armsceyes go - the person being fitted should rotate their arms and gesticulate to help find these key points. The top of the armsceye goes right where the humerus attaches into the shoulder girdle - it can be felt with a forefinger, if you prod in a little. There's a teeny dent. As for the backs of the holes, there will be a place where the flesh and bone just bumps up, when you have the person reach backwards and upwards. Inside that point, the ribcage is smooth and stationary; and then there's the bit that bunches. Put the armsceye just on that line where the fabric stops wanting to be flat when you move. Same in front. For the neckline, go with whatever you're comfortable with, with two caveats - the shoulders should be at least an inch or two wide (to take the pressure of the boobage-compression), and mind the nipples :->
Now, unpin it and lay it out flat on a table. Have a sewing gauge or ruler or calibrated piece of cardboard ready, with two lengths marked. This will let you mark your seam allowances on your patterns. 5/8 of an inch is the usual seam allowance, but allow at least 1.5 inches on the vertical seams, and on the shoulders; preferably more like 2 inches. Trust me, if you change shape much you'll want it there for alterations - no sense having to toss a great dress just because you gained ten pounds, or had a kid! And most of your shape-changes will generally be in the vertical seams. Take your gauge or ruler or cardboard and go around the edges of each piece, making dotted lines out where the seam allowance ends. If you don't have enough fabric, you can cut this one out on the exact seam lines (the pen marks) and put it over a piece of paper or another piece of cloth and make the second thing the true how-to-cut-it pattern. Or you can, as my guildmistress does (Costuming-Laurel Approved and Good for You!), attach more fabric to the edge of the cut with, of all things, masking tape. Just don't ever ever forget the seam allowance and sew off your exact-seams one ... as they say, been there, done that, got the t-tunic.
A wonderful idiot-proofing technique (I should know, I've screwed up more constructions than I care to count) is to draw on these finished pattern pieces in the way that a commercial pattern is marked - CF --> and CB ---> for Center Front (boob edge) and Center Back seams. I also generally scribble 'armpit' really small on the armpit edges. Do whatever works for you. Just be sure when the pieces are off you you know which one goes where (BOY did I mess that up the first time! :-) Developing a good system for marking up your pattern pieces can be even more useful if you also use the flatlining technique.
Briefly, flatlining is cutting out multiple layers of fabric, and making a fabric sandwich, as it were; fashion fabric on the outside, and the rest within, where they don't show. This has several advantages, all of them lying in the 'sturdiness' department. One, if your chosen fabric is particularly flimsy, this lets it stand up to wear, tear, and boobage. Even with heavier fashion fabrics (like upholstery brocades) the multi-layer technique does make it behave better. And for those of our sisterhood in the D+ range, a self-supporting bodice should almost always be made of at least three layers. The outer layer, the fashion fabric, is cut on the straight of the grain, as is the inner layer of flatlining. The middle layer is cut on the true bias (45 degrees to the straight of the grain; if | is the straight grain, when you cut this one the grain should be /). You can add additional / | pairs if you're using more than three layers. The rotation of the biases ensures that it won't stretch in any direction from the formidable boobage contained therein.
In any of these cases, once you have the layers cut out and laid flat together, sew around them in a simple line, about half the width of your sewing machine foot *outside* the seam allowance. This ensures that this 'basting' or what-have-you won't show when it's constructed. Be sure to press after this basting, and if that causes puckers to occur, rip small sections of the basting thread, re-flatten, and re-baste. This makes certain they're all acting like one happy pieces of fabric, and not working at cross-purposes.
The reason flatlining can help you idiot-proof your construction is that if you make your innermost layer an exact copy of your pattern muslin (with seam lines and 'armpit' marked), you'll never ever lose track of what goes where. :-> Of course, anyone looking at the inside of the garment will see it (unless you do a full lining afterwards) but I actually find that to be a feature, because then I can spread the flatlining (and idiotproofing) gospel by example, as it were. I also like to date my garments on the inside, just for my reference.
For a five-part bodice, just pinch the fabric twice in front, forming a center panel and two side panels. For an eight, pinch it once in the center, and once on each side.
Was this clear? Please feel free to email back with questions or whatever.
Eloise, hight the Eager, of Tree-Girt-Sea.
Lady Nastasiia Ivanova Medvedeva