After frosting the cake with the base coat of frosting, chill in the freezer for 20 minutes (or more in if refrigerating) until the frosting has firmed up.
Create impressions in the chilled, firm frosting using leaf cookie cutters. (Doing this with chilled frosting will give you a cleaner impression).
Using a small round piping tip, (tip 3 or 4), outline the shapes with buttercream. Next, go in and pipe your contrasting colors next to the previously piped outline(s).
Using a small dampened paintbrush, pull the piped buttercream towards the center of each leaf. I kept the outer outline of the leaf mostly intact.
That's it! You can experiment with the length of your brush strokes as well as thin brushes vs. flat, slightly wider brushes, etc.
It is a good idea to practice on a sheet of wax or parchment paper!
If at any time your base coat of frosting becomes too soft, you can firm it in the freezer again (15-20 minutes) as needed so that you will have more control over the texture of your brush embroidery that you are layering on.