Here is another tip on mouths............when you cut your foam or foamboard, draw the visible part of the mouth as you see it on your favorite muppet, then add about 1/2 inch extra all around the pattern. This will be covered up by the head/body fabric and glued in place with fabric glue or hot glue. It gives that nice finished edge around the mouth. My friend who makes puppets says that she ALWAYS uses black velvet for her mouth ...........because it gives depth to the mouth.........and because it is very durable. She makes 3D tongues out of card stock............and thin foam glued on. She takes scissors or a knife and carves a v-groove in the center of the tongue starting at the back edge and bringing it to the front tip of the tongue............then she covers the tonge form with pink or red spandex.........it's shiny like a wet tongue..............also when it is pulled tight it follows the contour of the tongue form and the groove shows nicely in the center of the tongue...........she glues the spandex to the underside of the card stock. Then the tongue can be glued onto the black velvet. Voila!!! Try it, you will amaze yourself.
Also make the black velvet cover the top and bottom of the visible part of the mouth.........this enforces the hinge area and just looks better than cutting two separate pieces of black fabric.
Now, hotglue your foam head form onto the back of the mouth............this essentially makes your puppet...........this is a good opportunity to see what the head will look like before you cover it with fleece...........allows you to alter the foam if necessary.........
Finally, add your head fabric over the foam and stretch the fleece over the lip area and glue inside onto the black fabric.............
make absolutely sure that when you glue the fleece that at the hinge area, the fleece lines up exactly from the top mouth piece to the bottom mouth piece.........this is what makes the mouth look so good. It gives amazing depth to the throat etc.........
If you make the edge of the lower mouth a different width than the upper mouth, it just won't look right.
Peace
Dave