I'm going to be installing yellow pine over the stairs inside my house. They are between two walls and I would like to put trim boards along the walls on both sides, down the stairs. How is this typically done?
I take it you're covering an existing set of stairs with the pine. In this case the trim board is put in before the risers and treads. If you are building the stairs from scratch, nail this board to the stringers before attaching the stringers to the walls.
If the stairs are in place you have to scribe each tread and riser and cut them out. You don't have to be too accurate here because the tread and riser will butt up against this piece hiding any gaps up to their thicknesses. When installing the treads and risers of pine make them tight to the trim board.
Wood has very little expansion on its end grain. Before installing hardwoods or finishes allow them to lay in the house for 48 hours to acclimatize to their new environment. Professional installers actually have a moisture meter to measure the amount of moisture in the subfloor as well as the hardwood.