Pretty straightforward... I found a cute, worn-out window from my garden shed and took out all the leftover glass fragments. Then I took a sheet of thin plywood, which my wife coated with an adhesive chalkboard film. After that all I had to to was screw around a bit(pun!) and apply some steel wire for hanging.
Don't mind the text, it's just showing how serious and mature I am all the time(it's in Finnish).