For TfL, you do nada, just wave your phone or watch over the gate thing and pass on through.
For payment (via Apple Pay, sure Google is the same), double click the button, late FaceID do it's magic and et voilà. For the watch, double-click the button and that's that, it's already on your wrist. Worst case now, I carry a back-up card and a £10 for prehistoric taxi man outside the station.