Long story short. I cut the PP into 1/2" strips, using 1/4" masking tape I tape the PP to a line where I want the covering to terminate. I then tape another 1/2" strip of the PP over the tape giving me the first PP and tape to cover over. Once the covering is ironed down, using a fine point marker, I trace the first tape edge, remove all tape and PP, cut the covering on the marker line and iron it in place. This is much easier and quicker to do than explain.
Another little FYI. Painters plastic works better, it is flexible, and easier to see where it is going being it is clear. I prefer the PP for protecting the covering when using an iron without a sock, especially the trim iron on edges or narrow strips.