The landscape editor Handsome Matt is talking about is only the first step.
Once you create a land you have to tell the game to load that land and provide code to allow the player to play on the land.
Here is the tutorial for how to do this second part:
This will get you started, to understand the scripting language to need to read the documentation which comes with the landscape editor.
Sadly texturing a land takes a more artistic and creative mind. For me it takes a long time and several attempts to get a land to look the way I want, even then I'm not always happy with my work, I can always think of something that could look better. I normally look at other lands and study they way they had textured their lands, the textures they used and the layer's properties, even the colours they used for each texture. Then I would replicate that on my land.
If you want to see another tutorial for texturing:
It would be helpful for me if your two images worked.