These are concrete poured into forms then I would imagine you could do stones or soil and sod, or gravel around them. You can see the forms right there. Could be a trex like board material (when you heat it in a big salamander space heater it makes them flexible) and them bound. Could be heavy cardboard like you see steep concrete forms.

This is Jenn Ski's house; if you follow the links it should bring you to her blog, or find her on Etsy and it will lead you over there.