Do you already have the skirt you want to dye? Does it have a label? What is the fabric content?

Cotton, rayon, or silk: find a large stock pot you can afford to trash (you shouldn't mix dye with food, even after washing). Put in at least 1 1/2 to 3 gallons of water, a full pound of salt (one round container), and either (a) a full bottle of black RIT dye or (b) two packs of black RIT dye powder. Bring to a boil. When it's bubbling, turn the heat off and move the pot so it's not bubbling anymore. Wet your skirt in the sink until it is all wet, then dip the fabric in, stirring with a spoon (also one that will never be used for food again). After a minute or two lift the fabric out of the water with the spoon, just enough to see the color. The wet fabric will be two or three shades darker than the final result. When it looks dark enough, pull the fabric out and take it over to the sink. Start by rinsing it with hot water, making it gradually colder. Rinse until there is no more color in the water. Then wash (alone, you don't want dye bleeding on anything else) and dry. If you have a washer dryer in your home, just leave the dye pot full until you are done. You can never be sure of the color until the skirt is dry, and you can reserve the dye to make it darker if it comes out too light. If it is too light, just re-heat the dye and repeat. Just make sure everyone in the house knows that the pot is full of dye and they shouldn't eat it!

Wool: Follow steps for cotton, but be very careful when stirring. Try not to stir the fabric too much, and use dye that is not quite boiling to prevent the bubbles from agitating the fabric too much. To rinse, try to submerge in a clean water bath that is sitting still rather than placing under running water. Fill, dip, empty and repeat until the water is clean. Then hand wash with gentle detergent and let drip-dry until fully dry.

Polyester, Acrylic: Make up a dye bath like in step one, but add double salt and double dye. Bring to a low boil, then add wet fabric while the pot is still on the burner. Be sure all the fabric is submerged, then add a lid (also not food-safe anymore) and boil. How long is kinda up to your fabric. If it's only partially polyester, it may be a short time. If it's entirely polyester it may never take more than a darker tint. In general I let boil at least a few hours, and have been known to boil for up to 12 for a mostly-polyester item. After your boiling time is up and you think the fabric is the right color, turn off the heat and let the pot sit until cool. This can be as little as an hour, but I usually let it go at least overnight, and have been known to forget about it for a whole day with no bad effect. When the pot is cool remove the skirt and rinse in the sink, then wash and dry as usual. This technique is really hit or miss. Either it works great or it doesn't work at all. If you can't get it to work, then your fabric has too high of a polyester content. You really have only two options at that point. You can either deal with the lighter fabric, or you can send it off to be professionally dyed. There are professional companies out there that have access to dyes that will bond with polyester. One I've heard of is ...

Also, here are links to a few messages on the Madam Malkins list about dying:

http://groups.yahoo.com/group/MadameMalkins/message/3122
http://groups.yahoo.com/group/MadameMalkins/message/3059

You may have to join to see them but it's easy if you have a yahoo id and you can always set your options to no messages or just un-join once you're done. But you really don't want to do that because they're a great group to get HP costuming information!

Alley Cat Scratch's LOTR site has a great page up about dying fabric as well. You should check it out! They have a huge archive of tips as well as some great links down at the bottom.