Well, I have a recipe that I think works with decorating. I've used it all my life, but I don't have exact measurements, so you may want to experiment with this on a small scale first.
Ingredients:
powdered (confectioner's) sugar
butter
milk
vanilla
(add cocoa if you want chocolate frosting)
Use as much powdered sugar as you want there to be frosting. Say you have a cup of the powdered sugar....you may want to try 1/2 stick of butter, melted. Add vanilla to taste (about a teaspoon) and maybe 1/4 cup of milk. Mix. You'll be able to tell if you have the right amounts if it turns into a consistency you like. If it's too pasty, add more milk (and butter). It's easier to add milk to a dry mix than it is to add more powdered sugar to a wet mix, so go easy on the milk.
I wonder if Kefir (a runny yogurt) would work instead of milk? My friend just told me that it works as a sub. for milk in pancake recipes. See? I may have to do some experimenting myself.
Good luck. It's a super easy recipe and it may just turn into your favorite too. :)