If you already have a Fly.io account, all you need to do is sign in with flyctl. Simply run:
fly auth login
Your browser will open up with the Fly.io sign-in screen, enter your user name and password to sign in. If you signed up with GitHub, use the
Sign in with GitHub button to sign in.
Microsoft WSL users may need to run the following command to make this work:
ln -s /usr/bin/wslview /usr/local/bin/xdg-open
Whichever route you take you will be returned to your command line, ready to use Fly.io.