You need the official Rollei rectangle converter. Not made anymore, you have to locate one on Ebay. When they show up, they are usually many $$$$'s.
I think Hasselblad made one too, but the Rollei one is better because it rotates and you can do horizontal or vertical with the same gadget. On the Hasselblad you have to buy either a horizontal one or a vertical one, and carry both. Those are really hard to find, especially the vertical ones.

I've heard that you can accomplish almost the same thing by just using a rectangular easel when printing, haven't tried it though. It seems like you'd get excess light all over the place, and I can't find my photon duster.