Your frock probably looks something similar to this right now (I know this doesn't look too attractive at this point). We are about to make this a lot better by simply adding a waistline.
1- The first thing you need to do is try on your frock inside out and take a piece of chalk or ballpoint pen and mark at center front, center back and each side where you want the frock to be cinched in. The easiest way to do this is to put on a belt where you want the elastic to go and just make a small mark underneith the belt at the designated spots.
tip - Since I am pregnant my waistline will hit just below the chest. If you are not expecting you may want to lower that waistline a bit so you don't look like you are pregnant.
2- Take a ruler and the same pen(make sure it won't bleed thru onto the right side of your fabric) or chalk and connect the dots around the entire circumference. (make sure this line is visible as you will be sewing right on top of it and will need to be able to see it clearly)
3- Take your elastic and cut a piece that is long enough to comfortably fit around the area you will have your waistline hit on your dress plus 1" for overlap.
4- With your frock inside out, pin one edge of the elastic to one side seam over the drawn cinch line and make a couple of stitches to secure in place.
Change stitch to a zigzag (the zigzag needs to be wider than the elastic as you will be stitching to either side of it, not on top of it).
Begin stitching a zigzag that hits on either side of the elastic right on top of the marked cinch line. Stretch the elastic as you sew and this will cinch up the fabric behind it. Continue to sew and stretch the elastic until you sew the entire circumference of the frock. Secure the other end of the elastic right on top of the first end. (sometimes this is confusing the first time you do it, but once you get the hang of it you will love it and use the method a lot in the future.)
Here are a couple of pics of the right side of the frock once the waistline is done:
Great job! It should be looking a lot better now. Next time we will hem the bottom.