Place the baseboard trim in the room where it is going to be installed and leave it there for a week. This allows the trim to expand or contract in accordance with the room’s temperature and humidity.
Measure each section of wall that requires baseboard trim using a measuring tape. If you have outside corners to work with, add the width of the trim to each segment that meets another at an outside corner.
Cut sections of baseboard trim that match the measurements that you found in Step 2.
Mark each segment of baseboard trim using a pencil, noting which piece of wall it belongs to and in which direction the miter cuts should be made at each end.
Cut the ends of the baseboard trim to 45 degrees using a miter saw. At this point, they are ready to be mounted to the wall.