Bake cake in the ugly sweater cake pan.Let it cool down and to room temperature.Then, place it on a large serving plate.
Use a spatula to frost your cake in white buttercream icing. Don't worry about frosting it neatly. This needs to be a crumb coat.
Fill 4 piping bags with 4 different colored icings and attach Tip#12.
Start piping an inverted/upside down "6" all over the cake, as shown by the arrows.Fill all empty space by piping random lines or squiggles.
For the sides, just randomly squeeze your piping bags in an angle and let the icing flow.It will look really messy but don't worry. It will get cleaned up at the end.
Now, position your toothpick in the center of each "6" and drag it out, as shown by the arrows. Do this for all the "6" shapes you made.
For the sides, drag your toothpick right and left randomly until you are happy with the design.
Use moist paper towels to wipe off all that messy icing around your cake.
Arrange some red m&m candies along the sleeves and the neck line.
Toss some colorful sprinkles around the base of your tie dye shirt cake. Enjoy!