Let's put our snowman onto the grass. It doesn't really matter what texture to use, we'll just use the grass one because our snowman is quite weird already, so adding grass doesn't change a lot :)
So we'll need a grass texture. We'll use this one as an example, but you are more than welcome to use absolutely any image you want - doesn't matter.
To assign our grass image to the floor plane, we first need to make a material for that plane. We have already assigned materials to all the spheres, but not to the plane. Let's make a new material by clicking the "+" button in the scene materials list:
You will see the new material is added to the list. Double click it to rename it to the "Floor" and then drag it onto the bottom plane in the scene:
Nothing has changed yet, as the new material is pretty much the same as the default one that Owlet uses for unassigned objects. Now make sure that the "Floor" material is selected in the scene materials list and look at the materials editor at the left:
You can make a preview of the currently edited material by clicking the "Refresh" button next to it (note that you can also choose the shape to use for preview):
Both bottom plane and preview are default-white, so let's make changes.
OK, time to finally change something by ourselves, right? Click the "Layer" item in the tree, then locate the "Reflection" control and click the white rectangle to change its color. Pick the color you like and click "OK":
Great, the bottom plane is now orange. Now let's drag our grass texture image onto the checkered square next to the orange rectangle:
As you may see, the checkered square got the "artwork" icon and the bottom plane got the grass texture. However as it still has orange color, it is not that visible. Let's change the color back to white to see the real texture:
So that's how it works. The main color of the material is called "Reflection" (cause you see things because they reflect light). In most cases all you need is to put a proper texture to the reflection slot.
Some materials have several layers, in that case you can start with the "Diffuse" layer which should work in most cases.
Update: since version 1.5 Owlet has slightly different materials and each layer can has diffuse and specular options at the same time. If you use the newer Owlet, you should use Diffuse slot for textures, instead of the "Reflection" mentioned above.
OK, you know how to do basic materials editing, how to assign images and how to change colors. Not bad, let's make the very last step in out introduction guide: