I have made a couple. It carves pretty easy with just the usual tools. Pick up a couple of cheap hacksaw blades to saw out the general shape. Then switch to cheap carving knives to get your finish shape. A cheap set of junky carving chisels will allow you to more detailed carving. Pick up a couple small cheap files at a pawn shop or flea market...mighty handy for creating flat surfaces. Then finish with little pieces of sandpaper, going finer and finer to work out the carving marks. Once you have it finished, heat the stone slowly in a 180 degree oven for about 10 minutes and rub beeswax into it.
Notice how I keep harping on cheap tools? That's cuz this stuff will ruin good woodworking tools. Wicked sharp tools are not a necessity for working this mudstone/Catlinite. If you have a mortar and pestle, save the chips and bits, as well as the dust. A little work with the mortar and pestle and it can be turned into a flour consistency. You can mix it with a binder like hide glue as an earth pigment.