You arrive in Brisbane and head through customs/immigration and collect your bag.

As soon as you walk out the glass door head right towards the rental car desks and in the far corner are a couple of Qantas check-in counters and Virgin ones as well.

Check-in here and they will take your bag, give you your boarding pass and a train or bus pass which you can use to get the domestic terminal. The train/bus pass is free so make sure to ask for one otherwise be prepared to pay around $4 to travel between the terminals.