I used about 1/2 yard of main print and a 2 1/2" x 42" solid for ruffle, as well as 19" of 1/4" elastic and a few inches of bias tape.
I used a shirt that fit my toddler as a guide for the pattern. I cut it so it was a-line, not like my shirt, but whatever. My measurements are for a 3T.Unfold it and cut one arm off at a slope, like so:Cut a second piece and sew them right sides together along the side seams.On the left side of the shirt (it will have the strap) use your bias tape to finish the armhole.Serge the edge of the 2 1/2" strip, then fold it over and sew at 1/4" .I used my ruffling foot, but use whatever method you like. With right side of solid to wrong side of main fabric inside the neck hole, sew together, ruffling the solid fabric. Serge seam together when you get back to starting point.It will now look something like this:Pin ruffle down (as a side note, this is where I realized I picked the wrong pink. Bummer. Live and learn...).To make a casing for the elastic, I found my smallest safety pin and it would fir through a 1/4" seam, so that is the size of my seam. Topstitch all the way around, leaving a 1" opening for threading elastic through. Measure the top of the garment, mine is about 26" around, and I opted to make my elastic at 75% of this measurement, so 19". Thread through opening. Sew elastic together - my machine likes to have fabric to hold on to, so I sew them together on top of a scrap then trim the excess. Topstitch opening. Serge bottom edge and fold towards inside to hem at 1/4". Done!For the baby, I opted to add another strap - I simply serged both sides of a 2" piece of fabric, topstitch the serging to the inside. With the tank on the toddler, I figured out where the strap should go and topstitched along the elastic casing line. Happy toddler and happy mom!