1. Using a food processor, puree enough strawberries for 1/4 cup of puree and set aside. (I assume you could probably mash them by hand as well)
2. With an electric mixer, cream the margarine. Once light and fluffy, add powdered sugar, 1/2 cup at a time, until well mixed.
3. Add vanilla and strawberry puree and mix until combined. Be careful not to over-mix here.
I used this recipe to frost 24 vanilla cupcakes. I had enough frosting left over for at least 6 more cupcakes. I recommend frosting something plain/less sweet, as I did, because the frosting is very sweet.
Source of recipe: This recipe was modified from: http://www.marthastewart.com/recipe/candace-nelson-strawberry-frosting [1]