The holes: we drilled each log with 20 holes, evenly spaced around the log, width 8.5mm if you’re using standard plug spawn – the diameter of the dowel plugs increases from swelling in the moist spawn environment.
If you’re using sawdust spawn, this might be different. You can get a small hand tool that injects a small chunk of sawdust spawn, for example, snugly into 12mm holes.