black long sleeve shirt
leggings or long johns
white fabric paint
glow-in-the-dark fabric paint (optional)
Step 1: Draw the bones
This really doesn't take artistic skill. There are lots of sites online that have anatomy graphics, and you can use these as a guide. Simple is really just fine. (Check out the skeletons in the daft punk Around the World video if you want to see what I mean.)
What you do want to know is about how big the costume wearer's bones are, you wouldn't want your skeleton's knee to bend mid- hip, for example. Have the person who will wear the costume try on the black shirt and leggings. Measure how wide and long the hips, upper leg, lower leg, torso, and arms are. Mark the wearer's knees and elbows with safety pins. Use these measurements to create your bones.
For bones that have a separate right and left pieces (arms, legs) you might want to draw one bone, then flip it over and use it as a symmetrical template for the opposite side of the body.