Installing the app is very easy. First go to the Overwolf Appstore and search for FirstBlood Royale, or use this direct link here
From this page download the app, install, and run for the first time. You should see the FirstBlood login screen like the one below:
If you already have a FirstBlood account, you can use your Facebook, Google, or Email address to log in and start playing right away! If you don't have an account yet, just select Sign Up to create your new account.