This is not a tutorial, but a wonderful inspiration for a baroque corrugated cardboard frame :)
If you like it more plain you can take another look at the corrugated cardboard frames.
Links:
Baroque Corrugated Cardboard Frame (Les cadres en carton) (French)
via: cadres en carton (cardboard frames)
Here at unikatissima:
Corrugated Cardboard Frames