Easiest way to do it is cream unsalted butter in a stand mixer and then add powdered sugar and beat till it's spreadable. A bit of vanilla and a pinch of salt are good, and for yellow I guess you'll want a bit of food coloring, too.
Recipes that call for powdered sugar by the cup are usually off--you'll need more than the recipe says--and anyway it's a PITA to measure. Much easier to add till it's thick.
Amelia, married to Bear since 12/31/04; our daughter Rowan was born 8/6/09; our daughter Sapphira was born 8/17/12