How do you enable a thank you page for any Pay Later membership? We'd like to redirect them to a page that tells them how to get us their contribution. Are thank you pages only displayed when using a payment processor? How do we display something as reinforcement that we got their submission?
"Offline" in civi can be a misleading term - it means a staff member has entered it in the backend, e.g. using the Memberships - New Membership menu choice. So you can't redirect to a thank-you page there since it's a staff member doing the entry.
If you mean pay-later, then when you configure the contribution page there's a checkbox for "Use a confirmation page?" at the bottom of the Title tab, e.g. at
civicrm/admin/contribute/settings?reset=1&action=update&id=1 (replace id=1 with the id of the appropriate contribution page). Pay later is an option on the Amounts tab.