now, take the O-ring ( of you chose to use one ) and glue it ti the inside of one bottlr top. you can press it down with the table tennis ball. ( make sure you dont have any epoxy glue on the inside of the O-ring )
wait for the glue to dry.
test the seal byholding the bottle top with the cap on the bottom, placing the ball on the O-ring and trying to suck air from the cap side.