To get the muzzle shape and size you want, hold up a piece of foam behind your mannequin. Mark where the vision and mouth is. Draw your muzzle. Using a box cutter or other sharp thin blade, cut your muzzle out. I'm using 2.5 inch thick foam. Flip it to the other side and trace the wearers face. Cut out.
-What you are doing essentially is making the muzzle form fit to the wearers face. After this you will need to bulk of the muzzle slightly to be proportional to the species. Be sure to hollow out the muzzle! I like to taper mine up to the bridge of the wearers nose for maximum airflow.
BIG TIP: MAKE YOUR FOAMWORK SMOOOOOOOTTTTHHHH. This can be done with little little snips. The smoother and more even your foamwork is, and more solid, the easier it will be to fur, trim, and finish later on. Fill in any gaps as well.
You can attach your nose before or after furring, I've done both. I still like to pattern the nose to fit the muzzle before furring. Yo can do this before or after carving.
Carving requires patience, frequent checking, and a pair of sharp scissors. Little snips. Round the edges and follow your lines.
For the cheeks and eye brows, same method as the muzzle applies. Holding up an inch thick piece (or 2 inches and cut it in half once you have your shape you want). Carve and attach. Brows are the same way.
At this point I like to fleece any exposed foam. Lips, inside of the mouth, seams, and lining areas such as the eyes or around the nose (to be furred over).
For eye shape I use the very base layer of foam cut out (seen bottom left). Find the eye shape, color the buckram and highlight, install, and test fit.
A video of the jaw test can be seen here. You may need Facebook to view.
Wow, this is REALLY great!
I actually ended up building my suit a similar way!