At that time Supergirl would have a different costume every month. Mike Sekowsky was writing and drawing the book, maybe editing it too, I don't know. Mike always had an eye for hip fashion of the day and he brought that to Supergirl. Other designs were submitted by readers. Some were good, some, not so much.
The costume pictured here is the one most people remember. I don't know who designed it, but it was first drawn by Bob Oksner in Adventure Comics #410. They still experimented with different costumes for a few more issues, but they came back to this one. How could any fanboy complain about the hot pants costume? Topping the deal was Oksner being able to draw the cutest girls in comics. His Supergirl flew with a grace that hadn't been seen before or since. Art Saaf, the other artist sharing drawing duties on Supergirl, was also known for drawing cute females with impossibly long legs. It was a good time to be a Supergirl fan. :)
This was done in ink on 11" x 17" Canson Bristol Board for our own Royce Thrower. Thanks again Royce!