scribing cuts

We are renovating our old Queenslander big time.
Bought new timber cornices, rounded (like the plaster ones) instead of the old square ones.
We cannot for the life of us work out how to do scribed cuts.
I mean actually physically start from scratch.
We have installed the square cut sides and are going around in circles trying to work out how to do the scribed ones.
Whatever we try to do, it does not fit
We know how to do the square ones but cannot comprehend how to translate it onto the rounded ones.
We have a compound sliding mitre saw and have tried to cut from just about any angle we can think of.
Anyone here who can (and are willing to) explain it to me in plain English?
TIA
Wolffie