L.L.
I make buttercream frosting pretty much exclusively, because it's so easy...and like Bridgett, I've never heard of using shortening.
My recipe calls for 5 T. butter (or margarine), softened, 3 c. powdered sugar, a few T. of milk or enough to get the consistency you want, and flavor to taste. If you want vanilla buttercream, use vanilla. If you want orange, use orange extract, etc. I make all sorts of flavors. Sometimes, I go really indulgent and skip ALL the flavors...just sugar and butter. ;) Remember, if you use an extract or vanilla, you'll need less milk. Put your extract in first, and THEN add the milk.
This frosting will get a bit crisp when left out, but it certainly doesn't taste old. If you want firmer frosting, use less liquid.
(Whoops, neglected to add how to actually make the frosting. Cream the butter and sugar, add the liquid and continue to mix until smooth.)
ETA: I'm sure this is one of those things where there are 100 different ways to make it, but I want to assure you that 5 T. is absolutely enough, and this recipe comes from my tried and true Betty Crocker cookbook that I have had forever. It's the only book I use, on the rare occasion that I need to look something up, because it's a great teaching cookbook. Also, it's absolutely not necessary to beat your frosting for 10 minutes...that's probably why it looked curdled. You're basically trying to turn the cream into curds and whey. You just mix it until it's smooth...if you have creamed your butter and sugar properly, no issues.