The image is from ShaderX6, if you're interested. The basic idea is that for each cascade, you partition the viewing frustum using a depth range and then build the 8 corners around the slice of the frustum representing that particular partition. Normally you build a light-space AABB around those corners to get a tight fit, but with stable CSM you use a bounding sphere instead (which you can actually do in world space instead of light space, since it doesn't matter for a sphere). Then you also do a little math to round off the translation of each partition, so that they only move in texel-sized increments.