Choose what animal you want to care for. Then continuously feed it (by tapping on it's food) and make sure your animal NEVER gets too hungry. I will eventually add more things like games, water, (Which I've JUST added) sleep and a 'fun' variable. I added a bite AND 'glug' sound so that it is as if the Fox is VERY thirsty.
I used this tutorial for some things: https://youtu.be/irhNLRWwhv0UST