Blender. How to cut and key a model for 3d printing

The previous video on this subject used the bool tool addon and although it is a slightly easier method, booltool does not work reliably for this in Blender and above. It does work but a lot depends on your geometry and what type of cut you want to do. This method does not use booltool and therefore slightly more complicated but not worryingly so. Previous version is here 0:00 Annoying but short intro 0:06 Plane cuts 6:52 Organic cuts