I was inspired by an old version of GW's How to make terrain.
(It is a GREAT book; ignore the paint trends and colors/ just focus on the concepts. In this book all the buildings are made of corrugated cardboard and PVA glue; it makes it very accessible and easy to start a project. No plastic card or other stuff needed). In this article they built an old temple that I used a the basis for my ruin.
I tore up an old cardboard box and made a few walls. The posts were made of balsa wood. I anticipated that the cardboard base would warp, so I glued down a few pieces of wood, which would keep it level. The wood scraps also serve as the broken stone floor. I finished the walls with some spackle.
To finish it up I found some rocks in the yard and positioned them nicely. I had an old tree laying around, so added that to one of the sides for some balance.
After priming in black I sprayed some textured paint on the walls to make the dry brush easier.
After flocking and adding pebbles, I chose a reddish color scheme like in the original book article. It took in total no more then 3 hours for start to finish excluding drying time.