The frosting is lovingly piped on using a pastry bag.
Like this one (okay so this exact one).
It’s very simple, rather like milking a cow.*
I stand the pastry bag up in a large, heavy mug to fill it. This really helps.
Lovely!

*Who are we kidding: I’ve never milked a cow! But Samantha learned how on a Sex and the City episode once and I think of it every time I do this…