it is not too tough with the right tools.
actually the easiest way IS to cut at 45 degrees, BUT you have to hold the crown on the base and fence like it will be on the wall, only upside down.
i am assuming you are using a power miter saw, if not, get one or borrow one.
there are 2 ways that are common; one with a compound miter , forgot the angles, but i can get them if you need.
the other way, as i stated above is to treat the base (table) of the saw as if it was the ceiling (or top in your case) and the fence as the wall, then just 45 it.
let me know if you need more info/help