Do you mean the charcoal? You can buy it already bagged and then I just put it where the water will flow through it. So at the outlet from my bog or tucked into the waterfall. Depending on how large your pond is and how your water flows, it could take a while to clear, but it definitely works.
Here's what I order:
I've also used sodium percarbonate for the same purpose. It's a powder that you can use to clear algae off rocks and pond surfaces, but I've also noticed when I use it, my water clears from tannins. (My water is pretty much lightly stained most of the time - I always have something falling in that seems to add to the slight tea coloration. I don't mind it so usually I leave it alone, unless it gets too dark.)