It comes with 100 download credits built-in, each good for one sheep. After they run out you can get more credits with an in-app purchase. After downloading the initial flock, you should get a few more each day.
The app is specially designed to minimize power consumption. With normal use it should have no measurable impact on the battery. It has an option to only download when on a WiFi network, in case you have a limited data plan.
Once you install and run it, tap the screen to bring up the status overlay, or hit the menu button to see the main commands, including the preferences where you can change the speed (FPS).
By default the speed is set to slow so it’s not too distracting and doesn’t use too much battery. If you want it to go faster, use the preferences.