Step 1. You can use the whole image or crop to square for better symmetry depending on the image.
Step 2. On a strong graphic image you may want to sharpen at this point to get crisper lines. Duplicate the Background layer by highlighting and hit Ctrl/Cmd J.
Step 3. Highlight new layer and Edit > Transform > Flip Horizontal.
Step 4. Go through the blending modes on your Layers palette. Darken, Multiply and Soft Light are my first choices to try. This nautilus shell actually used Lighter Color.
Step 5. Above all, experiment and have fun!