1. Prep your frosting. You need enough frosting to cover the entire cake plus enough to have the colored frosting for decoration. I used three colors; red, white and blue. So I made a batch of vanilla buttercream and separated it into three bowls to color and then put into piping bags.
2. Frost the cake in whatever flavor you want to use. It doesn't have to be white frosting, but a lot of the colors add a little bitterness to the flavor so it's better to cover the entire cake with a base of the plain frosting first rather than covering the whole cake in a colored frosting. (Since I didn't actually have a project going to use it on, I just piped frosting onto a circle of parchment paper for my base.)
3. Pipe the colors onto the cake in a swirl, one at a time. (Another cool design for this decoration is a star, but I would suggest practicing how that looks on a piece of parchment paper before trying it on a cake - it's good to give it a run through to make sure it looks the way you want it to.)
Don't worry about making this perfect...just get the general pattern you want. Follow your first line with the others you want to use - if you are just using two colors, it works best to alternate and have more lines of colors than to just have two (like blue, then white, then blue, then white). My three colors aren't really enough - four or five lines give you a better pattern.
4. Get a clean brush. I got a cheap little set of these at the craft store a long time ago and store them with my cake materials, designated for food use only.
5. Use the brush to gently push the frosting lines down and together (don't pull the frosting to mix the colors, just try to make the height of the different colors about the same and less separated so they will blend well when you pull across them in the next step).
6. Use the brush to gently pull a line from the center out to the edge of the design. Repeat this all the way around your design.
7. Once you are done pulling the lines through, you can go back and clean up a little bit if you feel like there are areas that got a little muddled. But less is more with this design - don't get too worried about cleaning up or making it too neat. Sometimes the continuous messing with it will make the colors sort of muddy and every time I try to go back and do much clean up I regret it.
See? Easy and quick, once you get your frosting prepped. Good luck with your cake, Grace!