Measure your lengths for the first boards to cut on the top AND bottom of bathroom on all four walls. You can remove your painter's tape at this point.
I learned that my walls are not all created equal, they have settled and have warps that I didn't notice with my naked eye. Measurements I took on one side of the wall would be different than the middle of that same wall, so my boards were all cut at different lengths!
Use miter saw to cut lengths of wood, cutting corners at a 45-degree angle with the saw.