The trick is to minimize the lossage from the chainsaw kerf, and this is achieved by dissecting the stump into pieces that can be milled with a standard vertical bandsaw (usually 24 inches).
It appears that a stump is sliced as a face cut, “vertically”, parallel to the trunk:
Rather than build a chainsaw mill, we can make face cuts, two feet apart, small enough for the mill to process thinner slices.
Once the stump is dissected into two foot high chunks, the mills precision saw can cut them into half inch face cut slices, and after drying, they can be planed into 3/8 inch panels.
Building a chainsaw mill
Another approach is a chainsaw mill, but it’s wasteful because of kerf lossage. You start with a single chainsaw cut to get a level surface. Once a flat plane is achieved, these experts built a chainsaw jig to slice the stump into level pieces.
BC woodwork shows how to create a chainsaw mill:
