Here is one of my terrariums being constructed.

You start with a glass bowl. Like this one!

 You drop in a layer of sand.

 Then you put in some blue colored sand so the terrarium can have some sand layering.

 Then put some more sand over the blue sand.

 Then put some river rocks over the sand.

 And decorate!

Here is how it looks.

A red road stretching threw the terrarium.
A little mat of moss.
A small air plant.
A rock with a little panda on it.
And a small shell!