Without turning your mixer on, use it to stir the ingredients together (to avoid a cloud of powdered sugar).
Turn mixer on to High speed and mix for 5 - 7 minutes.
Notes
This recipe makes a nice "thick icing". I added 1 - 2 teaspoons of water with my coloring when I needed it to "flow" a little better.
This recipe makes a large amount. Cover the bowl with plastic wrap to help prevent it from drying. Mine started to crust over, but I just mixed it together really well and kept going.
60 servings is just a guess, it will depend on how many separate colors you use and how much you thin the icing out.