As above, when you check in, if they give you bording card for next leg, you should get a card giving you access to the lounge.

When you get off the plane, go on the bus, it will go to Economy firsty - STAY ON THE BUS, we made the mistake of getting off and almost missed our connection. The bus will then go on to Premium terminal, there they will check your bording pass/invite (in fact they check your ticket anyway no-one asked us for our invite card.