As you do.
Our friend recently acquired British citizenship, so to celebrate I decided to make him a Union Jack cake.
I'm not very good at icing decorations usually, as fiddly things tend to be too much of a faff for me. In this case, it was simply a matter of covering the cake with ready made rolled out icing (painted blue), then rolling and cutting strips of red and white to make the crosses.
Ok, so it doesn't look professional, but it was certainly striking and you can tell what it was supposed to be, which for me is success enough!