UPDATE: Some pics from WonderCon 09!
Step 1: Materials
I've found this fabric to work well for the body of the costume. It's soft, breathable, and light enough to move around in but with enough weight to hold the package together. The downside to curly fleece is it can shed quite a bit when it's fresh, so it gets rather messy. However, a good thing about this is that the fur on the fabric was so long that it easily hid the many safety pins I used. Another good option is something called Flurr (fleece + fur). You want about about 2.5-3x whatever your height is.
1-1.5 yds of Black Chiffon/Mesh
This is the mesh-like material that will form the mouth. It is important that the material be see-through, since this will be your window to the outside world. The mouth will probably consist of 2-6 layers of this material, depending on the specific fabric you have. The most important part is that if you hold it up to your face, you can see relatively clearly through to the other side (usually there is a light source outside of your costume), but no one can see into your costume through the mouth (no light source within your costume).
Contour Shaped Sponge
You can pick this up at your local Home Depot for a few dollars. A good half-oval shape is what you want. The dimensions I found were 7.5"x3.5"x2.5", and I eventually cut in half length-wise however. This will form the lip.
Styrofoam Spheres (x2)
For the eyeballs. 4" in diameter will do, or 5" if you want to go bigger.
This will be essential to keeping the costume on your head. Pick out something that is light, nothing too thick. You don't want to start sweating too much while wearing the costume. It should have enough structure to it though stay firm and not flop around.
This should match the color of your costume. Thin pipe cleaners work well, you just need to buy enough of them to twist around to make a solid and flexible pair of antenna. Some glittery pipe-cleaner is also helpful in complimenting the solid colors.
Hot glue gun
Needle and thread (or staples)
Safety pins (various sizes)
A friend (very helpful to have someone to wear the costume as it is being created)
Step 2: Measuring the mouth, part 1
Center of the top of your head
Eyebrow line (approx upper edge of mouth)
1 3/4' from the brow line to the lower edge (of the mouth)
These will be markers for cutting out the mouth hole. The dimensions of the mouth hole are approximately 1' in width, and 1 3/4' in length, oval shaped. Mark the left and right edges of the mouth hole as well. Your eyebrow line will be the the approximate upper edge of the mouth. Be sure these markers all line up symmetrically with the length-wise axis of the fabric.
Next, cut out the mouth hole. Better to under-estimate here, you can always increase the size of the hole with more cutting. Keep in mind that whatever you imagine the hole size to be, leave a small 1" buffer between the edge of your cut, and the true edge of the mouth. This is because you'll need that flap to sow on the mouth covering.
Post Note: I recommend making the upper area of the mouth slightly more square than rounded. This is so that you will have enough room to have a bit of peripheral vision when you look through. If you don't manage this, you can widen the width of the hole by fastening the fabric to your beanie accordingly.
Step 3: Measuring the mouth, part 2
A good test of the visibility is to hold the total number of sheets close to your face and use a take a picture of yourself (with flash on) from several feet away. Your silhouette should not be visible, or only very slightly. At the same time, you should be able to see through the fabric and walk around a dimly lit room at least.
Step 4: Sewing the mouth
IMPORTANT: Be sure you're sewing the mouth fabric to the inside of the body, not the outside.
If you're not comfortable with a needle and thread, ask a friend who knows how to help you. Stapling is also an option, but that depends on the thickness of your mesh mouth fabric. After you've sewn the mouth fabric on to the body fabric, you can trim the loose flaps to clean up the area. From the outside it should look like a nice gaping black hole.
Note: A commenter suggested using hot glue for this step, which probably would make things alot easier. Other commenters have used velcro strips to make the mouth area detachable. This sounds like it would be especially helpful if you want to be able to show your face easily without taking off the whole costume.
Step 5: Fitting the beanie
Step 6: Creating the jaw
Once the jaw is attached, you may notice the mouth fabric feeling a bit floaty. You can weigh it down to give a it a better sense of being a "mouth" by pinning a small weight to the inside of it to create a sort of "throat". A knot of left-over fabric works well for this.
One tip, when holding up the jaw in the "closed" state, your eyes should fit just between the tip of your jaw and the upper edge of the mouth. This way you can still see without your Yip Yip opening his mouth.
Step 7: Creating the eyes
Test the eye position by placing the costume on with the beanie a comfortable and even place, then look if the eyes appear to be straight and level. When that is taken care of, cut out a pair of circles for the pupils out of black paper (mine were about 1" in diameter), and glue them onto the Styrofoam balls.
Step 8: Creating the antenna
When you have the finished antenna, affix it to the head area with safety pins, just behind the eyes. The safety pins can go through the beanie for extra support. Adjust the antenna so it stands up straight when the costume is worn normally.
Step 9: Finishing touches
With the sides sealed up, you can get to work on the front and back edges. What I did here was cut vertical strips up length-wise like a flier with tear-away phone numbers. I also cut out strips from left-over fabric and attached it to the ends. The more the better, it will help to cover up your feet and legs. Not too long though, you don't want to trip over it. The idea is just to give the costume that ghosty-alien look. You can either hot-glue the extra strips on, or use safety pins, whichever you prefer.
That just about wraps it up. If you need to adjust how the costume fits, you should be pretty comfortable by now with how much flexibility you have; I used safety pins pretty liberally as they aren't easily seen anyways being fastened on the inside. When not wearing the costume it seems easiest to handle it with your hand inside the beanie like a puppet; this way none of the headgear flops around too much. Feel free to give me feedback, or share any ideas you've added to your costume.
yipyipyipyipyipyipyip...uh huhh, uh huhh