I followed Inigo Quilez's article on terrain marching:

http://iquilezles.org/www/articles/terrainmarching/terrainma...

Be sure to browse his other articles:

http://iquilezles.org/www/index.htm

and his awesome video tutorials (which tend to be more about the math than the code):

https://www.youtube.com/user/mari1234mari

Enjoy!