I need to unlace a set of wheels so that I can repaint/powder the rims but I've never unlaced, or laced, a wheel before. I'm normally using Tuff wheels.
Hoiw do I do it?

There are a few websites that can give you good step by step instructions. Its not too hard but your first wheels will take a long time. But its worth it.
http://www.gsportbmx.com/tech/guide_wheelbuilding.php

OR another way is
1. Tape your spokes together in pairs where they cross/touch. Also mark the spokes either side of the valve hole so that you remember when replacing the new rim.
2. Undo all the nipples and remove.
3. Remove the rim. All the spokes will be left sitting/hanging in their normal positions.
4. Line up the new rim with the spokes-make sure the valve hole is between marked spokes. Tighten up the spokes EXACTLY the same amount each but leave them loose.say 2 turns each-Follow the instructions on the wheelbuilding websites to do the rest.
Hope this helps.
Some people will tell you not to try it at all. I think if you are good with your hands and mechanically minded-AND have plenty of time, try it.

Try
http://www.gsportbmx.com/ask/index.php#q7

Agreed, but it maybe worth getting a junker wheel or two to give it a try initially, just so you don't end up doing your first wheel then go and put a nasty gauge through the fresh powder. Personally when i stripped down wheels to relace, I use two methods.
I don't know ive never tried his method but just sounds little confusing but then again ive never tried it with tape. Anyway I either do a rim transfer line each rim up next to some other rim then move one spoke over to the other rim and rethread couple turns, go onto the next spoke and so on around the wheel keeping it as a wheel just don't bother tightening anything down until going back.

Or I take them out separating the spokes set them into 4 groups inbound left flange, inbound right flange, outbound left flange, out bound right flange. generally i number them 1,2,3 and 4 and just follow the instructions as shown on the gsport website so the same ones are in the same part of the flange although maybe in the wrong hole, more a problem when you have big gouged flanges like i frequently do.

By the way you'll need a spoke key to do so, don't bother avoiding it. you'll most likely need 15 or 14 gage on most bmx stuff for a few dollars you can get a cheap one with most main sizes on it, probably your best bet.

Anyway good luck with it and hopefully you'll do a better job than who ever built the wheel i just got today.

Here's a good beginner method. Start by zip tieing the spokes together everywhere they cross. Using a spoke wrench, take the tension off the spokes working around gradually half turn at a time. Start at the valve stem hole for a marker and work your way around. You will think a half turn isn't doing much but trust me, you do not want to start undoing spokes all the way especially if there is a lot of tension, you could bend your rim. By the time you get most of the way around (usually down to the last 4 or 5 spokes)you will notice all the tension sudenly goes away. Then after you go one complete revolution doing a half-turn on each, you then can just undo them from then on completey because you are safe from warping your hoop. Might be kind of a wrestle carefully bending and removing the spokes from the old hoop, but look how easy it will be putting the new rim on(if thats where you are going next). If you are going to just take the spokes out to paint or polish the hubs then skip the zip ties. There ya go brother. Hope I helped.