Use the large portal in the center of the Citadel to visit a new world. Each level has a Rick to defeat in order to earn a badge. There will also be multiple Morty trainers to fight, and a secret item cache in random locations. Morty trainers will only challenge you if they see you. You can sneak around using the environment if possible. Because of the random layout of a world, it is possible to reach Rick without encountering any other trainers by remaining stealth. To return to the Citadel either lose a fight to travel back with Birdperson, or defeat that world's Rick and a return portal will appear.