Plug your servo into port D3 on the Monster M4sk. We'll use the one-sided servo horn, and place the servo flat along the back of the mouth so that the motion of the servo horn will open and close the mouth.
Press the horn in place but don't screw it permanently onto the servo just yet -- we'll adjust the placement of the horn for optimal mouth movement first.
Your servo placement will depend on your design and sizing. The idea is to attach the servo firmly to the back of the face, with the swivel placed right at the open/close fold line.
As you're looking at the back of the face, you'll want to place the servo on the left side of the cardboard backing, with the servo horn close to the bottom. Test and be sure the motion is correct before gluing down -- if you put the servo on the right side instead, it will reverse the motion of the mouth (closing when it should open).
I glued a small piece of craft foam to the underside of the "chin" beneath the furry beard. Then I added three pieces of thicker craft foam as a spacer, so that when the servo horn is extended, the mouth is fully closed. Here is a side view with the mouth closed:
The servo horn is (generously) glued to the craft foam spacers. It swivels up and to the left, which opens the mouth on the right side.
Once all this is in place and the glue is fully dry, turn Fizzgig on and press the button. This is a good time to pop the servo horn off and adjust its placement on the head of the servo, if needed. Once you're happy with the motion, screw the servo horn down tightly with the short screw that comes with the servo (not the longer screw, that may damage the servo).