Perfectly symmetrical shapes are created easily by drawing half the shape, duplicating that half, flipping it, and then combining the result. However, combining the path into a single shape requires precise alignment or the use of a little trick that overlaps the elements. As long as you know what to do, either of these is easy; if you don't know what to look for, however, the end result could be not-quite-so-perfect, and the effect can be ruined.
Once the shape is created, it is easy to store it in the current shape set. From there, it can be selected and applied as a custom shape in any image.