Simple but pretty much flavorless. Lightly mix vegetable shortening and powdered sugar together, roughly a 1:2 by weight. Thin with water as needed. This mix can be quite stiff so if you are using a small hand held mixer make small batches and don't put off deciding to add water to thin or you will burn out the motor on the mixer.
You can use butter instead of the vegetable shortening if flavor and mouth feel are important. I prefer a mix of cream cheese and butter and as I have noted in another post I use lime juice for flavor enhancement.