i just posted this as a comment in another forum, and figured it might be worth reposting here.
these instructions are gearedtoward large guys; i'm very small and i pattern off a wife beater instead of a tshirt; i end up with something that has lines more like a sports bra, which is nice because it means a lot less of my torso is swathed in material, and because i can actually wear a wifebeater without anything over it. i'm lazy, but eventually i'll stitch a binder into the neck and arm holes of a beater so there's no constant adjusting to keep the binder covered by fabric. anyway, here:
email spandexhouse.com and ask for a sample card of the powernet fabrics.
when you get it, identify the heaviest one and order some of that.
for someone bigger, i'd say you should, as mentioned above, pattern off a tshirt. mostly what you'll use it for is neck and arm holes. get one that is about as small as your friend can get on. cut the sleeves off, and then split it up the sides and at the shoulder seams.
measure your friend's chest under the breasts, at the fullest point, and on top. for a begining measurement, average those. adjust the sizes of your tshirt pieces so that the side to side measurement of the front plus the side to side measurement of the back equals the number you got when you averaged the chest measurements. BE SURE, HOWEVER, THAT YOU LEAVE SEAM ALLOWANCE. that means: the pieces will need about half and inch extra on each side. so, if the measurements you get are 30, 36, and 32, the average is about 32.5. you want the finished size of both panels to be 16.25. so you would cut them to 17.25, which gives you half an inch of seam allowance on each side of both front and back panels. also add a half inch seam allowance at the shoulders, which you will likely just have to do when you transfer the pattern to the powernet, because you won't have extra fabric in the shoulders.
once you have pattern pieces of the sizes above made out of the tshirt, cut out one back panel and probably three, possibly four front panels from the powernet. draw a line up the sides and across the shoulders where the seam will go, which is to say half an inch in from the edge. line up the edges and the shoulder seams, and stack the pieces so that the back piece is sandwiched between front pieces. for example: front piece, back piece, front piece, front piece. be sure the shoulders and the sides are lines up on all pieces, and pin them together. when you put the pins in, do it so the heads stick out the sides and are parallel to the bottom (in the case of the side seams) so that you can stitch over them with the machine.
set your machine to the widest zig zag stitch possible, and the finest stitch count (most stitches per inch). use the line you drew as a guid for where to stitch. backstitch all your seams at the begining and end, which means stitch forward half an inch or so, then stitch backwards over that stitching, then stitch forward again for the length of the seam. don't tug on the fabric anymore than you have to; the feed dog will do most of the work for you, but it might get a little hung up once in a while. go slow, and be patient, and be careful.
when all your seams are stitched, you sort of turn it all inside out by folding the single back panel over to the side with the rest of the back panels. this hides your seams and extra fabric between layers of fabric.
try it on at this point. if it seems to work, great! if it doesn't, figure out where your problems are. is it too big? too tight? how do the arms and neck fit? adjust those things-- if it's too tight, you may have to start the process again and set your seams wider. too loose just means you put in new seams that are closwer together. you can cut the arms and the neck to get them where you want them, but be sure to restitch the tops of any seams you cut, including backstitching.
when you have a good fit, you can leave it like that without finishing the arms and neck hole. that's what i do. i think you could probable just stitch around them with the same zigzag stitch, but i haven't tried that. if you know someone with a serger, that should work, too.
i don't know how clear this is, but i hope it helps.
ps, white is probably the worst color for binders. spandexhouse offers a sort of caucasian "flesh" color as well as a solid black. of course, neither is going to be a perfect match to anyone's skin tone, but darker skinned people would do well with black and lighter skinned people would do well with the tan; in both cases the color will help hide a binder under a tshirt.