Ask HN: How to setup your application to accept SSO (SAML) from enterprises?

There's a bunch of services like Auth0 and OneLogin that purport to help with this, but there's a bunch of complexity simply understanding what to do and how things fit together.

If I as a developer want to enable an enterprise to login to my app via their SSO system, what is the easiest thing to do? Either FOSS or paid services are fine as long as they meaningfully make my life simpler.