If you already have a Fly.io account, all you need to do is sign in with flyctl. To sign in, run:
fly auth login
When your browser opens to the Fly.io sign-in screen, enter your user name and password to sign in. If you signed up with GitHub, then click
Sign in with GitHub 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.