For inside corners, one end of the crown is cut square and butts against the wall. The other end is cut mitered for an inside corner first and then coped. The process for measuring remains the same.

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But, unless the crown is too big to cut "standing up" like shown in the picture linked above there's no reason to cut it flat. The picture linked above shows how to cut crown "upside down and backwards" for lack of a better way to describe it.