Measure the walls and cut the base molding to length with a miter saw. This project used a polystyrene molding that looks like wood but is much less reactive to dampness or heat.
Cut the base shoe – a quarter-round molding that goes at the bottom of the base molding – to length and attach it with a finish nailer. Nail it at an angle so that the nail goes downward, into the wood. The nails won't hold if they're driven straight into the polystyrene molding. Miter the corners as with the base molding.