Start a new game and enter a name. After the FMV sequence ends, hold
Triangle + Square + Circle. A menu should appear. Release the buttons and
check your inventory to find all kinds of items. Feed your Digimon with
something. If he shakes his head, quickly press Triangle + Square + Circle
again. If he eats it, quickly press Triangle + Square + Circle again.
Depending what was fed, he will turn into different kinds of monsters. If
you are not satisfied with that monster, hold Triangle + Square + Circle
immediately after the monster changes and feed him with something else.
Release the buttons when you are satisfied with your monster. When you're
satisfied with your monster, stop holding the three buttons.