You will need to know your OpenAthens domain name. This is usually the same as the scope registered against your domain organisation as seen on the organisation summary.
E.g. if your OpenAthens domain is institution.ac.uk, your SAML 2 metadata address will be:
The metadata address should be sufficient for most SAML targets, however some may instead want you to specify endpoints, certificates and other data manually instead. If they do:
Endpoints / SSO address:
This will be the x509 certificate in the metadata, topped and tailed as follows. This is sometimes called PEM format.
This should be 'Redirect' rather than 'Post'.