Whether you loved them as a kid or hated them for giving you nightmares, there's no denying it's fun to go around freely and anonymously shouting "yipyipyipyip" to those around you. Here's how to make a costume based off the Martian Yip Yip aliens from Sesame Street.

UPDATE: Some pics from WonderCon 09!

http://www.flickr.com/photos/sliny/sets/72157614507235315/
http://www.flickr.com/photos/gpbmike/sets/72157614506357137/

Step 1: Materials

5-6 yds of Curly Fleece
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.

Beanie
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.

Pipe Cleaners
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.

Other materials:
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

Drape the body fabric over your head length-wise so it runs from your heels, over your head, and down to your toes. However, give yourself approx 1 1/2-2' of extra length for the front half. This is because you'll need the slack for when you lift up what will be the jaw portion of the costume to move the mouth. Duct tape is a handy measuring marker for this step. While draped in the fabric, mark these points on the fabric:

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

Next cut out a rectangular piece from the black mesh to act as the mouth fabric. Be sure it has plenty of space to cover the hole. Depending on the transparency of your fabric, you'll need several sheets to form the mouth. With my fabric, 3-4 layers was enough.

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

Here comes the tough part. After you've decided how many layers you want to form the mouth, you're going to sew them to the body fabric to cover the hole. It helps to tape the black sheets together first so they can be sewn on all at once. This part can get frustrating as each layer may move about on it's own, but just take your time. The good thing is, complete accuracy isn't important, from the outside, no one can tell.

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

You've probably noticed by now that when draping the body over yourself, it tends to slide around a bit. To fix this, you'll need to attach your beanie to the inside, right where you've marked the center of the top of your head. You can use large safety pins to get a rough position, then lock it down in place with smaller ones around the rim. When adjusting the placement, be sure your eyes are just beneath the upper edge of the mouth fabric, so you can see out through the mouth. However, you don't want your eyes to be too low into the mouth, or it may look like either your Yip-Yip is always looking down, or it's upper lip is wrapping around your forehead.

Step 6: Creating the Jaw

With the dimensions of the sponge I bought, I cut it in half length-wise to make it thinner (easier to handle). Mark an area a few inches past the lower edge of the mouth, and tape the curved face of the sponge onto the inside surface at that location. This is where all the movement will take place. Try moving the jaw up and down while looking in a mirror, to make sure it looks ok. When you're satisfied with the placement, hot-glue the sponge on.

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

While inside the costume, have someone else mark a good position for the eyes, just based on whatever looks best. Another option is to lay the costume out flat on the floor, then lay the eyes on the approximate area according to what appears symmetric. Once you have that, you can hot-glue the balls directly onto the fabric. I prefer to have the eyes touching each other, so I glue them together as well.

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

Take the pipe cleaners and wind them up into a single shape as seen in the images. I used about 24 total pipe cleaners to get the structure right, with more making up the base and less around the ends. When it was all wired together I lined it with four glitter pipe cleaners, just because.

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

While inside the costume, have someone start sealing up the sides with safety pins. Fold the flaps inward and close them up. The safety pins can be about 6-8" apart, going down the side. Just clean it up however you feel comfortable, as long as you can keep others from seeing into the costume from the sides. Don't close it up too tightly though, you need space to move around, and to maintain the Yip Yip look. It doesn't have to be super neat, the Yip-Yips have a pretty sloppy look. And in the end, you're still essentially just wearing a sheet over your head. (Commenters have opted to sew up the sides or use velcro strips, both which work fine. Safety pins are probably the fastest option. In any case, you may want to leave a hole for you to let your arms stick out in case you'll need them)

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