Begin by clearing the floor along the wall. Then, lay a plastic sheet protector on the floor so that the longer side of the sheet is parallel with the baseboard.
Hold your paint roller sideways so that it’s vertical. Place the roller onto the sheet protector on the floor, and slide the roller to the wall so that it's touching the baseboard.
Roll the paint roller across the baseboard, pushing it slightly downward so that the roller stays in place on the sheet protector and catches any excess paint.