You'll need a large black robe or cape. Button or tie it down the middle, so what you wear underneath doesn't matter.
You can use DIY Fashion's tutorial to Make a Hooded Cape.
You can make a Death eater mask that looks like the ones from the movies, or you can come up with your own design.
You can make a mask that fits your face perfectly with just a little newspaper and some glue by using DIY Fashion's How to Death Eater Mask tutorial, or the Death Eater Mask tutorial here on The Leaky Cauldron.
After you've made your mask, it's easy to paint on your death eater designs.
Any thin stick can be made into a wand. Peel off any bark and use a piece of sandpaper to smooth down the sides of the stick. For a pointed end, use a small knife to whittle the end of the stick. Shaving the end of the stick should be done with adult supervision. Apply a dark or light wood varnish to your smoothed wand.
You could also use about.com DIY Fashion's Easy Pencil Wand for kids, or one of the many wand tutorials available on Leaky Crafts in the Woodwork and Other Crafts sections, or spring for a toy Harry Potter wand.
Wear a pair of black slacks, or make your own simple black drawstring pants using the tutorial available on about.com in their DIY Fashion section. Wear a pair of hefty black boots. A pair of black leather gloves, or plain black gloves, are needed.
Slick your hair back with gel and keep the hood of your robe up to draw attention away from your hair.
This tutorial was taken with permission from about.com's DIY Fashion section.