To sign on, start with your non-dominant hand horizontal, palm facing down, and your dominant hand up at a right angle. Lower the dominant hand so that it becomes flat on top of the non-dominant hand.
It is like you have a switch and are lowering it onto the on position.
Use the on and off signs together to make a game out of it. Get a toy that lights up or plays a song. Switch it on and do the on sign. Switch if off and do the off sign. The on and off signs also pertain to the prepositions, as in putting something on the table, or taking something off the table.
Opposing conceptual signs, like on and off, are more easily taught as pairs. They also lend themselves more to teaching in the real world than through the flash cards. Switch a light on and off or turn a fan on and off.