To convert a Solid into a smesh, just toggle the option in the 3D properties of the object. When converting from a Solid into a smesh, a set of facets will be generated to become facets on the smesh. This can result in unpredictable results/
The following is an example of going directly from Solid to smesh.
You can improve the predictability by first converting the Solid to a TC Surface then to a smesh. The following is an example of going from Solid to TC Surface to smesh.
Changing between Solid and smesh for 3D primitives like a box will generally produces more predictable results as well.