The Invader deck goes on the gameboard on the “Explore” action space. The Invader deck is 12 cards:

5 Stage III cards on the bottom, then

4 Stage II cards, then

3 Stage I cards on top

Mix up the cards from each Stage and choose without looking at them, putting unused cards back in the box.

(There are 4 Stage I cards, 5 Stage II cards, and 6 Stage III cards, so randomly return one of each to the game box.)