Before starting on the watercolor design, your cake should be assembled, frosted and thoroughly chilled. See this post for tips on How to Stack, Fill, Crumb Coat and Frost Layer Cakes.I assembled and frosted my cake with the base coat of buttercream at night, and refrigerated it overnight before decorating it the next morning. If you don't have that much time, make some freezer space and freeze your cake for 1-2 hours so that the base coat of buttercream is very cold and firm.
Divide the 3/4 cup buttercream into several bowls, and tint each portion with gel food coloring. You can use as many different colors as you like, depending on the look you're going for.
Place the chilled cake on your cake decorating turntable. Randomly dab the tinted buttercream around the cake.The less buttercream you add, the more textured your final design will be, and the more buttercream you add, the smoother and more blended together it will look. Note that I barely added any buttercream during this step, and my design has a nice textured look to it.
Now take your bench scraper and lightly sweep it around the sides of the cake as you spin the turntable. You should just barely skim the surface of the cake as you're smoothing it. If there are any gaps you want to fill in, simply dab more buttercream on that area and smooth it again with your bench scraper.
When you're happy with how the buttercream is blended, fit a piping bag with tip #2D, fill with buttercream, and pipe a few drop flowers onto the cake.
Let the cake warm up at room temperature for 3-4 hours before serving.