I bought these two shirts for 33c each at a yard sale. The tank top was horrendous. Absolutely terrible. It was shiny and made of some weird slinky material. Just...ugh. So I made this shirt for my office instead!
Cut the ugly tank top into strips of fairly even sizing. (It doesn't matter if they're not perfect.) Set them aside.
Pop the buttons off of the button-up. (It gets way too hard to sew over them otherwise.) Resize to fit if needed.
Pin the strips of ugly tank top to the inside opening (where the shirt used to close.) Fold them so that they ruffle nicely when you hold it vertically.
Sew over your pins (or pull them out as you get to them, whichevs!)
Try it out! I suggest belting it now that the buttons are gone (or you could reattach the buttons I suppose but that seems like a heckuva lot of work!)