Press the green flag to start the game. Keep pressing the character until you find the one that you like. When you find it, press the space key. Do the same thing to chose a pet, except press the left arrow key instead of the space key. You can see how hungry and happy your pet is in the top corners of your screen. If your pets hunger level gets up to 20, or its happiness gets down to 0, then it will stop the game. You can make it less hungry by pressing the red circle at the top of the screen, and you can make it happier by pressing the blue one. You can stop the game by pressing the right arrow key.
I learned how to make a vertical pet from a Scratch Team video, and I added my own "touches."