I had taken a picture of this spirit house at "Gallery in the Garden" event at Woodstock, IL which is much loved and well attended. http://www.galleryinthegarden.org/
I have tried to show how I painted the straight edges of the wooden post. Make sure the surrounding paint is dry and then tear and stick a piece of drafting tape to create the edge. Now you can paint freely and then lift the tape. You'll get a clean edge. Please stop by later to see the completed painting.