We already had this cute Lamby suit, so that's where my decision to become Mary came from.
I sketched out an idea of what I thought Mary would wear (there's really no set costume).
While at the thrift store I found this Hawaiian dress and this blue robe. I liked the material that blue robe was made from because it was stretchy knit and I wouldn't have to have finished edges.
First thing I did to the white dress was just sew up the sides, since the XL dress was too big for me. I pinned where I needed to take it in, cut off the extra, and sewed it back together.
For the blue robe, I just cut out the pieces for the top of the apron and bottom-- sewed them together (unfinished edges). I used the straps from the robe to tie around the back. Lastly I sewed straps that went on the top of the apron and crisscrossed in the back.
That was it! Simple, semi-homemade costume.