My suggestion: Look at some references. Waterfalls, cliffs from other games, whatever floats your boat. Look at them for a long time. Now, take that image that is in your ref and try and make it into something 3 dimensional in your head. Think about what goes where and how the surfaces interact with each other. THEN start modeling.

There will not always be a tutorial made to help you. Sometimes you need to sit down and just figure out how things work before you start working on them.