I know how to scribe a pattern onto fondant, wondering how to scribe a pattern onto buttercream?

It depends on how intricate the patters is. There are several methods. The easiest I think is to trace the pattern onto waxpaper , using a pencil, then place that on the cake and trace over the lines w/a toothpick.
Another way: cover pattern w/waxpaper, then pipe royal icing; after it dries, lay it on the crusted b'cream cake and press the royal into the b'cream which will leave an impression to be followed.

Once you have the pattern the way you want it on a piece of paper, place it on top of a piece of cheap foam. Use a straight pin and poke holes following the pattern. The side next to the foam will have a roughness to it where the paper pokes out from where the pin pushed through. When finished you will press the rough side against the crusted bc and lightly trace the pattern with you finger or a ball tool. Repeat all around the cake where needed. You can then pipe over the impression to create the pattern. This is just one way to do it. I laminate my paper pattern then prick the holes that way I can wipe it off and reuse it and save it for another project. Hope this helps.

thanks much guys!

The pinprick method is excellent. I've been using it quite a bit lately.