The other day I had to scribe a shelf to go on top of a plastered wall to mark the transition between that wall and the stone wall above.
|A well scribed shelf!|
That meant scribing it to the stone work.
I use a pair of compasses to mark mine, setting it to the widest point and then marking the board from there before cutting it with the jigsaw. As I work on old buildings a lot, it's a job I have to do quite often so over the years it's become easier and easier - although I challenge anyone to get it right first time!