Glue the button in place temporarily to hold it steady before you add sealant. I used superglue, (crazy glue - USA) hot glue-gun might be better still.

The button doesn't need to be in exactly the same place - the metal lever it presses underneath is quite big. And it doesn't need to be strong - just enough to hold it while the sealant goes around it and dries.

Add sealant as in step 12.