I'm currently putting up ceiling moldings made of some kind of cellular plastic. I didn't understand how to make them fit together in the corners! But then I found [link] and then it clicked! Now I don't really know what hålkälslist is, but the described technique worked perfectly for me. You just place the molding in the mitre box with the "wall side" of the molding against the side of the mitre box closest to you. The "ceiling side" of the molding is then against the bottom of the mitre box. Abracadabra, the angles were perfect! I hope my description was understandable.
Addition: I've now looked up what hålkälslist is and they resemble my moldings except for the material.
