I am posting this for PepsiTwist, who has requested a very detailed description of how to make a ruched waistband for the infinity dress.
Step 1) Okay, first you need your measurements. measure your waist, and divide the measurement in half, then decide how wide you want the waistband to be. (i like 4-6 inches)
Cut out two rectangles which measure as follows: the long side should be half of your waist measurement, minus 1-2 inches so that it's snug, since you are using a stretch fabric. The short side should measure your desired width PLUS seam allowance, about 1 inch extra. (piece A in the image)
then cut 2 more rectangles using the same waist measurement as the first two, but multiply your short-side measurement by 2 or 3. (by 2 will be slightly ruched, by 3 will be much more so, i suggest doing at least one practise version of your waistband so you know how ruched you like it.) (piece B in the image)
Step 2) Take the B pieces and sew (using a long stitch with loose tension) along both edges (not the side that is your waist measurement!!) of each piece. then pull the thread at one side, holding the other end in place. push the fabric toward the end you are holding steady, creating a ruched effect.
Step 3) Take one A piece and one B piece and sew them together along the top edge using a regular stitch at normal tension. (indicated by the coloured line in the image)
Step 4/5) Flip over piece B to the other side of piece A.
This hides your stitches, and it should look like this from the side:
make sure to do this for both pairs of rectangles.
Step 6) Press the top edge if you can so that it isn't rounded looking like in my image. I drew it like that so that it's easier to see.
Now take your attached pieces and lie them flat. line up the bottom edges. if B does not reach the bottom of A, loosen the ruching to make it wider. if B is longer than A, tighten the ruching to make it narrower. when they are the same length, pin in place, making sure the ruching is fairly uniform, and then stitch using a regular stitch at regular tension. (lines of stitching indicated by coloured lines)
Step 7) When you have both halves sewn together, lie them on top of one another making sure that the B sides are facing each other. pin in place, and sew along the edges (indicated by the coloured lines)
Step 8 ) turn inside out so that the smooth A sides will lie against your body and the ruched side face out. Now your waistband can be attached to the straps and skirt of your infinity dress. notice that the bottom edge is left raw while the top edge has been turned inside and sewn (in steps 3,4,5) the nicer looking edge should be at the top because it will be seen at your back where the straps are not attached, but the bottom edge can be sewn under when attaching the skirt and will then be hidden.