I started with a girls size L tank top that I got at a yard sale for 75 cents.
(A little steep, I know, but when I saw it I immediately had a vision of what I wanted to do!)
Turn the tank inside out and fold at center front.
Pin to create the right bodice width.
Before sewing, open to check that the strap placement will fit your baby.
(I just eyeballed it here.)
Sew only to the bottom of where you want the bodice to end.
Cut away the excess, leaving a seam allowance of 1/4" or so, then cut straight across, perpendicular to the fold.
Open the bodice, looking at the wrong side, and pinch down the excess fabric to create a box pleat.
I stitched this by hand, attaching it to a previous seam to keep it in place.
This is what it should look like.
Cut the straps off the back side of the tank.
Pin along the side seam, creating a shape you like while eyeballing the width it needs to be at the top so it will fit your baby. Ease into the existing seam to you don't have to mess with the hem.
Mine didn't match up at the top. Problem? No.
I scrunched it. ("Scrunch" is a very technical term!) This created more interest in the "bodice" of the dress.
Sew along where you pinned, then fold the finished side over the unfinished side. With pins, match up what you sewed on the first side, and sew the second side.
Finish the seams however you like (or not, knits don't ravel!), and....
...Ta-Da! A super cute little dress!
You could try it on your baby and mark where to attach the straps on the other side, or do like I did and use the straps as ties.