How to focus stack a set of images in Photoshop

I recently rented a Nikon 105mm VR Macro lens for a weekend, and wanted to experiment with different types of macro photography.

One of the things I was most interested in was focus stacking. See, there's a problem with macro photography in that you're dealing with a depth of field measured in millimeters when reproducing images at a 1:1 ratio, even stopped down to f/8 or f/11. And, wanting to avoid diffraction at higher apertures, there's no way to take a straight-out-of-camera picture of a 3D object that's sharp from front to back.

One frequent subject of my close-up photography is the Raspberry Pi single board computer. You can see the problem when taking just one photo: