I got this idea from another blog, but I didn't use a pattern or their tutorial so I don't remember where I saw it. If you saw something like this somewhere else, I'm not taking credit for the idea, just sharing my version of it.
My husband was getting rid of a shirt that didn't fit him well anymore. Aside from some wear in areas from age, the shirt was still good and I thought I could make something out of it. My first thought was a dress for my 2 year old but seeing as how I was swamped with Christmas stuff, I wasn't willing to commit my time to that.
Therefore we have the Shirt Bib.
All I did was get a bib that I liked the shape and size of and use it as a pattern. I pinned it to the front of the shirt where it buttoned and cut it with about 1/4 inch seam allowance. I bought a coordinating hand towel (probably $1.50) and cut out the same pattern. I stitched the two together (right sides facing each other) and left a 2-3 inch opening to allow me to turn the bib. Then I top-stitched around the whole thing which closed the opening and helped the bib to "sit" flatter when on. Because of where I cut the shirt, there was only one button attached so I cut off another one and it helped to keep the two layers from shifting. I cut some Velcro for the neck and sewed it on. The whole thing took a couple of days on and off (like 5 minutes here and there) and I think it looks pretty cute. :)
Sorry for the lack of step by step pictures. I wasn't sure it would work the way I wanted and didn't think to take any pictures as I went. But its pretty easy I swear.