Maybe you could cut cardboard into the shape you want and paint accordingly.
Hi, I think I saw that same picture online, because I was inspired to do that in my girls bedroom as well. I came across a project blog that showed how to make a scalloped painting stencil out of freezer paper. I haven't tried it yet, but I think if you just flipped the stencil over, you could get the look of the purple and white wall that was on that website. Here is the blog: sarahsprojects.blogspot.com/2010/01/painting-scalloped-trim- Happy painting!