How to enable fingerprint authentication for Google Play payments (Android Marshmallow)
Open up Google Play on your Android Marshmallow device, and tap on the menu button that you'll see in the upper left corner. Then, tap on Settings, and, at the bottom of the screen, you should see a "Fingerprint authentication" tab (with a checkbox on the right). Taping on that will enable fingerprint authentication for Google Play purchases on your device - you'll have to confirm that you really want to do that by entering your Google password. And that's all there is to it: you'll now be able to authorize app and content purchases just by using your fingerprint.