You don't actually need a pastry bag to pipe these, but it sure makes things handy. This mixture is too thick to pipe from a plastic bag (the usual go-to pastry bag alternative) so make your own with a triangle of parchment paper instead. Here's a great video on how to do just that.
'cake decorating' on Serious Eats
If you're into making dramatic decorations for cakes out of gum paste, you may want to consider making your own. This recipe requires a few special ingredients that are sold in specialty cake stores, as well as on the Internet.
This recipe is extremely versatile and can be used with any flavored extracts. Or, if you prefer to make a natural vanilla bean buttercream, use the seeds scraped from the inside of a vanilla bean. If your flavoring does not impart color, this recipe is great for adding color too. I recommend gel color instead of liquid food coloring, because it contains less liquid and you don't need much to get the results you want.