On the sign-in or sign-up page, click Login with Google. You are redirected to Google, asked to pick an account, and sent back signed in. If this is your first time, a new account is created using your Google email as the sign-in identity.
Accounts do not merge. If you signed up with an email and password and then click Login with Google using the same email, the two are treated as separate accounts — the Google sign-in creates a new account rather than linking to the existing one. Pick one method when you sign up and stick with it.