In this context, “flood” simply means to cover the surface of the cookie or cake or whatever with icing. If you feel confident and steady, you can flood each cookie just by using a pastry bag filled with the icing. I recommend using less icing in the flooding process than you imagine you will need, as it should spread to the outline and fill in the surface of the cookie on its own. If you're not quite confident using just the pastry bag, or if you want to be extra careful, squeeze a few dollops of icing over the top of the cookie and use a clean, dry paintbrush or the back of a spoon to gently spread it to the outline. We don’t recommend pastry brushes; craft-store paintbrushes with synthetic bristles work best.