Here is how I carved the scrolls on the horned plane (whose name is Sven, by the way).
It turned out to be much easier than I expected, so it's no wonder Dutch planemakers added them as a design element. It's a cool feature that doesn't take all that much time to make.
After I carved the shapes with chisels and gouges, I touched them up with files. I use a ton of files in my shop; they're invaluable with these types of projects.
I'm actually finished building Sven, so the three shots at the bottom of this post show a couple coats of boiled linseed oil worked into the wood and the carving I added to the top of the plane.