Summarize this article:

Place one of the cakes cut-side-up onto a pretty plate. Swirl some buttercream frosting onto the top of the cake. This time, set it cut-side-down. Use your hands to make the two layers even; you don't want any overhanging edges. Place the tip of the bag against the center of the cake. Using gentle pressure, begin piping out frosting while moving the bag outward in a circle. Think of it as drawing a tight spiral. For a simpler cake: plop icing on top, then use cake decorating spatula to spread it towards the edges and down the sides. Work your way around the first rose that you made until you fill the top of the cake. Continue making more roses along the sides of the cake. You should be able to make about 2 to 3 rows of roses along the sides. Go over your cake and check for any gaps between the roses. If you see any, fill them in with a small squirt of frosting. If you used a coupler, you can switch to a simple round tip for this step. You don't have to do this, but it's a great way to bring some color to your cake, especially if you spread the frosting instead of piped it on. Plus, your little one might like the texture when he or she grabs the cake! Instead of going for the basic rainbow sprinkles, consider using something more creative, such as silver hearts, gold stars, or edible glitter.

Summary:
Spread some buttercream onto the first layer for the filling. Place the second layer on top. Pipe a tight circle onto the cake to make a simple rose. Continue making more roses. Fill the gaps in with more frosting. Finish off with some sprinkles, if desired.