Can someone explain 1 pawn + king vs king stalemate to me?

Like, what situation is it impossible to stalemate vs a king and 1 pawn with only a king (how do I know when I've screwed up?), and rules for what I should do in order to stalemate. I keep playing against the CPU stockfish, but I keep getting it wrong some times and he checkmates me.

youtube the rule of opposition, it might help, but it would be nice if someone could explain it

Maybe this will help :
www.youtube.com/watch?v=LFRNFh3eyuo