Start at
https://sp1.zxidsp.org:8443/zxid
or
https://sp1.zxidsp.org:8443/zxid?o=E
If you had common domain cookie already in place, and you are already logged in the IdP, the SSO may happen automatically (go to step 3). The automatic experience will be typical when you use SSO regularly for more than one web site (i.e. several SPs).
However, if you get a screen titled "ZXID SP SSO", you need to paste the IdP's Entity ID to the supplied field and click "Login". If zxid SP already obtained the metadata for the IdP, you may also see a button specific for your IdP (and in this case there is no need to know the Entity ID anymore or paste anything).
Next step depends on the IdP product you are using. Usually a login screen will appear asking for user name and password. Supply these and login. You will need an account at the IdP.
For more slick IdPs, that's all you need to do and you will land right back at the zxid SP page titled "ZXID SP Management".
Congratulations, you have made your first SSO!
However, some IdPs will pester you with additional questions and you will have to jump through their hoops. A typical question is whether you want to accept a federation. You do.
Sometimes the federation question does not appear automatically
and you need to figure out a way to create a federation
in their user interface and how to get them to send you
back to the SP. Sometimes the word used is "account linking"
instead of federation.