1. Set Allow Refer a Friend to Yes (default). This adds the Invite a friend option to the customer profile page (and, per-customer, unlocks the Cashback % override field in the admin).
2. Enter Digital Amount to Referring Customer (credit the referrer receives) and Digital Amount to Referred Customer (credit the new customer receives).
3. Optionally set Show Invite a Friend in the Site Footer to Yes to add an Invite a friend button to the footer (hidden on cart, delivery, and confirmation pages).

Referral links use the format /register?ru=<customer id>; the site captures the ru parameter into the visitor's session whenever it's present in the URL, regardless of these settings — the settings only control whether the Invite a friend buttons are shown to customers, not whether an already-shared link works. Credits are granted when the referred customer completes their first checkout; both parties also receive the referral notification emails. Per-customer overrides for both credit amounts exist on the customer edit screen.