So I tried sketching something up for you, but it looks pretty terrible. So I will do my best to describe how I did all this.

1) The last bit of assembly of the box was the sheet of plywood on the bottom. To do this I just laid the box upside down, and screwed the plywood into it.

2) I then drilled holes in the bottom. This would allow excess water to drain from the bottom. The number of hols was somewhat random, I think I did every foot or so. But it is all symmetrical.

3) After I flipped the box back over, I jacked it up on a few 2x4s underneath (not just on the edges, but across the middle of it too). This aids in both helping the plywood support the weight of all the dirt, and gives space for the box to drain.

4) I layered the bottom with a fine screen type material. Think like a screen door. Something that would allow for water to seep out, but not the dirt from getting out.

5) A ~2" layer of lava rocks was placed on top of the screen. Lava rocks were chosen because they are porous, so if dirt were to get between them, water could still trickle through the actual rocks themselves. They recommended this at Home Depot.

6) I then poured the dirt I had picked out over the top while breaking it up at the same time.

Hope this helps. Let me know if you have any other questions.