You can layer your charcoal and then your gravel or you can do it the other way around. I've seen people do it both ways and their doesn't seem to be much of a difference in either outcome for the health of your plants. In this example I did the charcoal first and then the gravel.
Start pouring your charcoal until it just covers the bottom of the container. Shake it a little to even it out. Then add a couple small handfulls of gravel around the container to make sure you have and even coating around the container. Once you have just covered the charcoal you can now pour slowly from the bag until you have about half an inch of gravel on top. If you just pour straight on top of the charcoal to start, all the gravel will push the charcoal out of its way and you'll have an uneven layer of charcoal at the bottom.
Take some of your moss and just make a thin layer on top of your gravel. No too much though, you are only using it to stop your soil from sinking right into the gravel. Think of it as a screen.