Many ways to do this, and perhaps none of them are wrong. Illegal's link above is what got me started (thanks dude!), and I now have a happy DWC farm going. IT WORKS SO DAMN WELL.
Anyhow, here is my DWC cloner: I bought a "under-bed" plastic tote from Walmart - Sterilite #1860, 34"x16"x6". I use 2" netpots - it's really not too hard to slide the rockwool out once roots are formed, so I can even reuse the net pots. I used a 2" holesaw to cut the holes - 20 of 'em (you can use a smaller tote - just keep it near 6" shallow, or you'll have a lot of water to manage. Then, I use two cheap air pumps on a flexible air stone (those bendy snaky ones have inlets at both ends). No matter - just get some bubbles, not a lot are needed to fill the airspace with 100% humidity. Then, I bought a submersible aquarium heater (make sure to get one that can adjust its temps - I set mine around 80-85% in the beginning).
In the pictures, you'll notice that the net pots stick up above the top; simply put your clone into the rockwool (using your preferred rooting compound), and push the rockwool MOST of the way into the 2" net pot (I leave a little room at the bottom to keep the wool a little drier). So I fill the water to about 1.5-2" above the bottom of the net pots. The bucket is a good size for a humidity dome, but you won't need it for long. I recommend taping over unused holes to lock the humidity in, but I will open a hole or two to control this if the rockwool is too wet.
So what is too wet? If it's obviously moist on the tops, I think it's too much. Currently, I have the tops all but dry (touch 'em, and you can feel a little moisture), but the bottoms are fairly soaked/wet. This balance ensures that the wool has enough air, and that the roots will head down towards the moisture, instead of wandering like Moses til they hit the edge.
The photos show most of my holes taped up (only running 8 clones currently); also, notice the photos of the single clone - dry top, wet bottom.
Hopefully this helped - I know people have many other methods, but this is mine.