All you have to do is take a photo of your pet and open it in the app. I ended up using the comic setting which is the third setting on the bottom strip of the app.
You can also add some little clothing details to your pet which I did with a little bowtie for my Willy. Just use another image of the clothing item and apply it to your image.
After it is completely dry, take the white acrylic paint and fill in the remaining space around the image to make it appear to have a white background when turned around.