The basic steps are essentially the same for any symmetrical object. Once the symmetry is created and the shape combined, the shape can be stored.

1. Create a new image.

2. Set the guide(s) for the mirroring.

3. Create half the shape.

4. Duplicate the shape half.

5. Flip the shape half.

6. Align the 2 halves.

7. Combine the 2 halves.

8. Select Define Custom Shape from the Edit menu.

9. Choose the Shape tool.

10. Switch to the Custom shape tool.

11. Click on the Custom Shape Picker on the Options bar.

12. Choose Save Shapes from the Shape Picker pop-up menu.

In steps 1 through 7 you create the shape, and in steps 8 through 12 you permanently store the shape in a Photoshop library.