However, I must put in a disclaimer here: this method is perfect for costumes or less formal occasions, but I wouldn't recommend it for something like a wedding gown. To do that properly, you need fabric loops on the edges. As I didn't have matching fabric (and didn't want to take the time to make fabric loops even if I did), I went with buttonholes. It worked splendidly!
Also, don't try to change the dress more than 2 sizes. Sorry ladies, you just cannot make a size 4 into a size 12. :) The dress you start with has to fit you at the hips (because you won't be adding any extra width there), so a dress with a wider skirt works best. You can also start with a dress that fits you, you'll just do one thing slightly different, as I will point out along the way.
To do this, all you need is an old dress, and some double-fold seam binding to use as the tie. I went with some about 1/2" wide, but use whatever width sounds best to you.
Here is how the back of the dress started:
It's hard to tell, but there is an invisible zipper there. You can do this on a dress with an invisible zipper or a regular zipper.
Open up the zipper and rip it completely out of the dress. Be careful not to snag the dress in the process. It can take a little time, but it's mindless work, so turn on the TV and relax while you work.
Next, you will sew the two back sections closed. In this next picture you can see I've marked the area where you will be sewing (on the inside of the dress, of course).
Now turn the dress inside out to sew those two seams. If the dress is too small, like mine was, sew right along the original seam line, as shown here:
On a dress that fits when zipped, you want to sew inside the original line to open up the dress a bit and show more of the lacing. If the dress already fits you perfectly, sew a bit inside the seam line at the top, and taper it to be even with the original seam line at the bottom, as shown here:
(Please note the above picture is just a rough idea to show you the process, and the indicated sewing line is probably not accurate. Sewing about 1 1/2" inside the original seam line at the top will give you a nice starting place for your corset).
Flip the dress right side out again and give the new seams a good press.
I know you can't really tell the difference between this picture and the one where the zipper was still in. You can see the difference much better in person. :)
Mark some buttonholes along the two opening edges at even intervals. Make them for the width of the seam binding you are using to lace it up. Because my seam binding was 1/2" wide, I made the buttonholes to fit 1/2" buttons.
Play with it until you get a spacing you like. I ended up leaving gaps measuring about 1 1/2" between each buttonhole, because I wanted the lace to be slightly more spread out. You can make yours closer together or farther apart.
Sew a buttonhole at each mark on both sides of the dress opening.
Close up of the buttonholes:
The dress itself is done! To make the lace, keep reading.
If needed, join the ends of two packages of seam binding to make the lace long enough. These things need to be much longer than you think, and the more buttonholes you put in, the longer lace you will need.
To join seam binding, spread both pieces open and put right sides together in an L shape, then stitch it diagonally, like this:
Trim off the excess at the corner.
Fold the seam binding back together, and you have a neat diagonal seam.
(For a little more detail on this process, check out the end part of this post).
Keeping seam binding closed, sew along entire length of open edge.
To lace the back of your corset dress, start at the top and lace it like a shoe, crisscrossing the laces and always bringing the lace from the inside of the dress out at each buttonhole.
You're done! That wasn't so bad now, was it? Enjoy your new corset back dress!