Basically, you cut a hole in the middle of a square or circle of knit fabric and sew two 8-10" wide "straps" (~45-60" long rectangular panels) to it, adding a facing (a 2" wide rectangular tube the size of your waist) if you don't want the raw edge to show in the back. A small 1"-2" overlap of the panels in the center (where your boobs will be) is convenient.
Tips & Tricks
-This dress works better with lightweight super stretchy knits that have lots of give.
-Fabric comes folded in half lengthwise on the bolt. To find a square, fold it in half crosswise and then fold it at a diagonal. If the raw edges all line up with each other, it's a square. To make it a circle, measure the straight folded side (it usually comes out to be ~24") and mark the circle at intervals of 24" from the point with pins and cut. Leave it folded and cut the waist hole. For my 26" waist, the radius of the waist hole is ~2.5". Knits stretch so it doesn't have to be horribly accurate.
-Knits stretch. To keep the waist snugly fit, i stitched a piece of elastic to the length of the facing.