You'll need a large black robe. It buttons across to the right side, so what you wear underneath doesn't matter. Use this tutorial in about.com's DIY Fashion section to make the robe, only.
Although the robe pretty much takes care of any other clothing issues, 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.
Voldemort appears without any shoes, but a pair of hefty black boots is advisable. Even black sneakers will work in a pinch, or for children.
Use a sponge to cover the face in white to simulate Voldemort's pale skin. Add subtle hints of blue to tint the temples, jawline and a touch under the eyes. Apply a touch of red at the corners of the eyes, and under the eyes as well. Make sure you blend the red and blue into the white so that the tints of color appear natural.
Well, a lack of hair, really. Try getting your hands on a bald cap from your local costume shop. Otherwise, slick your hair back with gel and keep the hood of your robe up so no one is the wiser.
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.
Apply a set of fake fingernails.
If you'd like you can also repeat the make-up tips for Voldemort's face above on the backs of your hands.
This tutorial was taken with permission from about.com's DIY Fashion section.