I made this River Diorama years back, for use with 3 3/4 inch figures. The background are two print-outs of images I got from a face book page called "Living The Adventure Dios" which has a section titled "Action Figure Backgrounds". However in this I will only be talking about the base I made for the pictured diorama.
Step 1: The River Base Itself.
At the time I made this I didn't take any pics, so I will describe how I made it. These pics will show what it looks like.
I went to a local park that had a large lake by it and took pics of the lake to determine what it actually looked like: The water was NOT blue... Water is clear. But if there is enough water- As in a large river, lake, or sea,- the water scatters the blue light from normal light and appears blue. I observed the surface of the lake and saw that the top few inches of the water was clear, and the blue coloring began a few inches down. So I came up with a way to simulate it for a diorama.
The bottom layer is a large sheet of corrugated cardboard cut from a box. For the river I used a blue material called "Foamie" that I had bought much earlier: However blue posterboard will work as well. I measured then cut a single piece and glued it to the base cardboard. After it dried, I took some old cellophane I had and put it over the blue, then taped down the ends of it underneath the cardboard base. Thus I had my "River" with clear top layer and blue underneath.
I then cut a large section of foam board to fit the "Land" side of the base. I peeled off the paper on one side of it, then glued it to the base peeled side down. THIS was to make sure the foam core of it stuck and stayed stuck to the base. After it dried I peeled off the top layer of paper as well.
To make the "Grass" I use two items called "Moss Sheet" which is moss applied to a weblike base: This is one of the items used in making floral displays. I glued down two sheets of it onto the "Land" side edge to edge, then set it aside to dry.
And that is it. Making the diorama base is easy to do in itself or bought ready made. The two print outs are tacky glued to the upright back of the diorama and this was glued to the base itself.
So, this dio is where Luke and Yoda hang out, or where GI Joe and Cobra are having a battle, or a bikinied jungle girl is hanging out with her animal friends, or dinos are stomping around. It all depends on what figures are use in it.