First, took 2 pictures of a very co-operative subject
#1: Head in sharp focus
#2: Rear segment in sharp focus
This was achieved by shifting the camera slightly, so that the same perspective is maintained, but with a different part of the subject in sharp focus. Most guides would suggest this method instead of using the focusing ring because using the focusing ring would change the size of the subject, and change the bokeh, which would make a smooth stack more challenging later.
I tend to agree. even with a very straightforward pic, stacking just 2 shots, there were a lot of things that did go wrong. I had to use the healing brush to smooth out some of the more awkward areas that didn't stack properly. Fortunately, it was only very slight.
And Voila... the successfully stacked output:
K5, Vivitar 2x macro converter, Sigma 50mm macro, monopod, AF160 Ringflash