Step 1: When you first login to your account, it will give you the option to login with Face ID. Select the button that says Face ID. In order to sign in with Face ID, you must already have typed in your login information at least once. Face ID is a secondary, but faster, method of signing in.
If you want the app to default to this login method, select Save.
Step 2: The next time you login the following icon will show up.
It will scan your face and if it recognizes it properly, this next icon will show up and log you into your account.
You can now access your account by signing in with Face ID instead of typing in your login credentials each time.