You have a G25, so on your shifter push down on the gearstick (as in push it into the shifter, not down as in 3rd to 4th) then twist the grey knob anticlockwise (which will only spin one way anyway) whilst holding the gearstick down, once it clicks in, the paddles will work for shifting or just pushing the gearstick lightly up or down will also work. To swap back simply push the gearstick down again and twist the knob back clockwise.