I originally learned about this in the mid 90's in a manual for this drab brown and green Timex Field Watch that I bought. I wrote about this on my blog awhile back too:

https://brokensecrets.com/2010/04/22/use-your-watch-as-a-com...

North: hour to sun, bisect with 12 for south.

South: 12 to sun, bisect with hour for north.

Equator : mark the movement of shadows, I guess.