First, prep your ingredients and tools: divide the buttercream into four even batches, dying each batch with a different color. Fit 4 pastry bags with couplers, and fill each pastry bag with its own batch of buttercream. For one of the pastry bags with the petal tip.
Squeeze a tiny amount of buttercream onto the center of the flower nail. Pat a parchment paper square on top of the buttercream, pressing down and rubbing it across the surface of the nail to "glue" it onto the nail. Arrange the parchment paper so that it is relatively centered on the nail. Brilliant! You're ready to pipe flowers.
Start by piping a small blob of buttercream in the center of the parchment paper. It doesn't have to be a specific shape or even pretty — a small blob will do. Next, hold the piping bag at a 45-degree angle with the wide end of the piping tip at the bottom of the nail; squeeze the piping bag, turning the nail at the same time, so that it pipes a half circle around the blob of buttercream frosting. Pipe another half circle, starting from the halfway point of the initial half circle. Repeat until you've got a circle that forms the base of the rose. Repeat this process for the outer petals of the rose — as your flower grows, you'll notice that you'll need to pipe longer and longer half circles. Continue making the layers of petals until you've reached the desired size for your rose — smaller ones will look more like conventional roses, whereas larger ones will look almost like peonies, succulents, or even cabbage flowers.
Once satisfied with your flower, carefully remove the parchment paper from the flower nail with the flower attached and place it on the half sheet pan. Repeat the process for however many roses you wish to make, moving the petal tip to different piping bags for different colored flowers. Go crazy and make as many flowers as you want, but be sure to reserve some frosting for finishing the cake and gluing the flowers on to the actual cake itself. Once done, transfer the sheet pan to the freezer to freeze for at least 1 hour, or until the flowers are cold and firm to the touch.