I tried my hand at shirring, and made a lovely top. It is not her normal style, which is o.k. She chose cupcake fabric, very birthday-like.
It is made from 2 rectangles 27″ long X 35″ wide – she wanted it long enough to wear with leggings. The edges are rolled hemmed. 12 rows of shirring using elastic thread at 1/2″ apart. We used 1″ straps, just in case she wanted to wear it to school.
The following tutorials were most helpful in the making of this top:
I cannot find the link, but I recall that you should cut the width of your piece about 1-1/4 to 2 times depending on how full you want the garment. I used 2X as my amount here.