This is the image that tells the whole story to date.
As yet I have not carried the vinyl beyond having one foot in the doorway. No shuffling no heavy lifting. I have matched up my alignment with the doorway in two directions
The two lines match in this image. It looks a lit scewed, but that is because I have lifted the vinyl to enable cutting the corner.
The only shuffling left to do is to line up the underside with my third marking. EASY, Seems clear to me If a 76 year old can handle this stuff, Any handy D.I.Y. person can