There are going to be cakes on this site that require “piping” borders using a stiff buttercream icing. “Piping” uses a pastry bag filled with buttercream and an attached “tip” that makes different shapes and different styles of borders.
If you’ve never used a pastry bag before, it can be a bit intimidating… but it really (truly) is simple once you know the basics.
Here’s a link to a YouTube video that demonstrates how to add a tip to a pastry bag, how to fill it, and how to pipe a few simple borders. The only caution I will offer about this video is that the first several times you fill a pastry bag, it will be a lot messier than the delightful woman in the video makes it look. Go slow, make it as neat as you can, but don’t worry if you end up with buttercream frosting in your hair. Great art is often messy.
Once you understand how to fill a pastry bag and insert a tip, you might enjoy this video from CakeGenie on piping ten different buttercream borders with one tip.