First, cut off a tank top about 1.5 inches below the underarm seams.
Cut a length of fabric for the skirt which is several inches wider than your waist and hips (mine is doubled over here).
You could simply gather the top of the fabric all the way around but I pleated mine. The pleating adds a slimming element.
Attach it to the remaining portion of the tank top, taking care not to stretch the knit of the tank too much. (Obviously you'll still need some stretch to be able to pull it over your head.)
Once I got this far I realized it was too short.
Easy remedy: Add a panel to the bottom.
It's my go-to sundress. I need another one (or three) now. My 12 year old model zapped this one. Sheesh, she won't let me pick her clothes but she keeps stealing mine :)