This is the basic buttercream recipe that I use for piping flowers, I like it to be a little stiff because that would help keep the shape of the petals nice and sharp.
Beat butter until soft and creamy. This would take from 3 to 5 minutes depending of the mixer you are using. Then add all of the icing sugar and only mix until all combined. Add vanilla and mix, if the icing is too stiff to pipe then add milk or water little by little unit desired consistency is met.