The best tool to use for creating zest is a microplane. It creates a really fine grate and it's so easy to control the depth you go into the skin which means you get all the delicious orange flavor but no bitternss. If you don't have a microplane, use a knife to trim the top layer of the orange peel away from the fruit, but avoid cutting too deep and picking up the white part of the rind which is bitter.  After trimming it, you can chop it with a knife on a cutting board for a finer texture.