Step 3: Activated Charcoal Layer
The next layer to put in is activated charcoal or activated carbon - same thing. This is the stuff from your Brita water filter or fish tank filter - if you've got that, toss it in there. Otherwise, you can buy small amounts online, or skip this step entirely. I just happened to have a whole lot of activated charcoal lying around for a future project so we took a 1/4 cup or so and added it in.
My sister and I devised a simple cardboard chute to deliver materials precisely where we wanted them in the terrarium. The chute, plus a push stick or brush works much better than the dump method.
The activated charcoal layer just keeps things "fresh", or so I've read.