What's the actual process to create a photo merge? How many images and do you focus on different parts of the subject them merge them?

It's not really called "photo merge". It's called Auto-Align, and then Auto-Merge. Here's my workflow: Say you want to shoot a bunch of grapes. Use a tripod. Turn off your auto-focus. Focus on the grape closest to the camera, and mark that spot on your lens and focus ring. (white pencil) Next, focus on the grape furthest away from the camera, and mark that spot on your focus ring. Now make a mark half way between your other two marks. I usually make 5 marks for 5 exposures at f.16. Shoot a photo at each mark. Now load all 5 exposures into the same file, (keep the layers in the right sequence) select all layers, and choose Auto-Align. You'll probably notice that some of the layers are a little brighter than others. Adjust the brightness so that they all look about the same. Next, with all layers selected, choose Auto-Blend layers. Choose Stack Images, but uncheck Seamless Tones and Colors. Now, you have masks on all the layers. Delete the mask on the last layer. I usually blur the masks a little. Then, go into each mask and paint out any obvious mistakes the program made, with a soft brush. After doing this a few times, you'll be surprised at how fast the process gets.

Very nice result, Rimglow.  Which lens did you use for this?

REgards

Canon EF 100mm f/2.8 Macro USM.
 Can't remember if I used extension tubes or not, but I do occasionally.