I recently installed the SAML plugin in Jenkins and made sure everything was updated also.
I started to configure Jenkins within the browser console and navigated to "Configure Global Security".
There I found the option to select a SAML radio button which gave me configuration options similar to what this page will show you: https://github.com/jenkinsci/saml-plugin/blob/master/doc/CONFIGURE.md
I was able to find the idP file to paste into Jenkins by following the ADFS example posted on the SAML page https://github.com/jenkinsci/saml-plugin/blob/master/doc/ADFS_CONFIG.md
After adding the Metadata to the idP Metadata field and selecting the validate button it told me it was successful. I then clicked Save. I logged out of Jenkins to test and I've not been able to log in. Other users are also not able to log in. Attached is are a couple videos that demo trying to log in.
I have completed all steps on the ADFS example page except for the part labeled "Encryption: import key from JENKIS_HOME/saml-sp.metadata.xml file" and "Signature: import key from JENKIS_HOME/saml-sp.metadata.xml file".
URLs I have used or referenced:
SAML plugin page: https://plugins.jenkins.io/saml/
Configuring Jenkins: https://github.com/jenkinsci/saml-plugin/blob/master/doc/CONFIGURE.md