Posted toe adjustment confusion

I’ve read rodneys method of checking the toe in with a 2x4 about 20 times and still don’t fully understand it. Somehow what to do with the board escapes me.
Am I putting the 2’” piece long ways on the floor and sort of tenting the 4” section against the tire. Drawing a line. The getting my framing square and finding the front and rear of the tire.
I am actually really handy with this stuff but this really escapes me.