When you are learning to pipe buttercream, it helps to have a lot of practice! This buttercream is made with solid vegetable shortening and water so it doesn't require refrigeration and it will hold up to repeated use. This is the buttercream I use in class for learning to pipe flower on the nail.
Cream shortening with mixer on high for about 1 minute
Add powdered sugar, mix on low until sugar is incorporated
Add vanilla, if using, and water for desired consistency. Mix on high for 20 - 30 seconds, scrape bowl, mix another 20 - 30 seconds.
This is based on Wilton's Original Buttercream recipe.