I put a piece of cardboard inside the sock so any glue or marker would not go through. I cut a slit in the top of the sock as you can see above. When the kids are done decorating the sock cut an oval piece of cardboard, fold it in half and glue the sock to the cardboard. This will be where the kids make the mouth move. The inside of the "mouth" can be colored or glue some felt on it (see picture below).
The kids had a good time working on this and they spent about 30 minutes decorating it. Depending on how much glue you use, it could take up to 4 hours to dry.