take a large bandanna and cut it in 1/2 diagonally. round out the corner that will be at the neck and bind it off. that's what i did when my daughter was small. the other thing i did to avoid binding was cut a rectangle long enough to go from underarm to underarm plus four inches. measure it from under the breast to the height that you want it to be. add in one inch top and bottom. sew in a casing, top and bottom. thread through with ribbon or pretty braided fabric strips, etc. tie one around the back and one around the neck. the fabric will gather up in front. it looks very cute and you don't need a pattern.
(should i copyright that? hehe)