With a spiral slicer. The ham is put on a skewer and rotated slowly past a stationary blade parallel to its leading surface.
Sometimes some things are better off pre purchased but basically you want to cut through the meat at a slight (15 deg) downward angle starting perpendicular to the bone. start at the thin end working...
they cut them with this special machine that cuts spiral things. including ham. ;p
Some people have difficulty making clean slices of ham. Using spiral cut hams makes it easy to get clean slices.
wendell dunn invented the spiral ham technology