At Animazement 2011 I went as the Witch with a group of Left 4 Dead cosplayers. We were looking for something that would be easy and inexpensive to put together, and Left 4 Dead was a good choice in that regard.
Cosplaying in a group is much more fun than cosplaying alone, so pick a game or anime that matches everyone's ability level. We managed to pull this together almost the night before the con (not advisable), but it worked out in the end.
Parking garages make sweet photo spots for Left 4 Dead. I'm pretty sure there was a cop staring at us off-camera.
First I considered using aluminum foil and just wrapping them around my fingers and painting them black, which a lot of people do, but I worried that it would fall off too easily. Instead I made some origami claws and lengthened them with sections of cut cereal boxes rolled into points and then taped down and painted black and red. This is a good tutorial for the claws themselves.
I feel like the result turned out pretty well. The claws were easy to take off, but they stayed on very well when I needed them to and were very sturdy. Sadly I don't have a good close-up of the claws.
Pretty self explanatory, I just got a shoulder length silver wig and dipped the ends in grey paint to make it look kind of nasty and get that gradient that you see in screenshots.
Most of the costuming involved doing full body paint the morning of the con. I used a great makeup tutorial by Audfaced as a template and worked with that I had. Sadly, I don't have any good closeups of my face either (I hadn't anticipated making a tutorial at that point).
Here are some things I learned to do/not to do from this experience.
1-Unless you are very secure about your body, make sure you get a second opinion about how you look before you leave the house. Because I was covered in body paint and it kept me rather warm, I didn't realize that I actually looked very very naked until I saw the pictures afterward.
2-Bring body paint with you to the con if your costume requires it. I didn't take into consideration that I would wash the paint off of my hands every time I used the bathroom.
3-Be prepared to clean up a huge mess afterwards. Body paint is messy on a good day, and when you're rushing to get to the con paint will fly.