Open Keycloak

Select or Create a new Realm in KeyCloak

Copy the link pointing to the SAML 2.0 Identity Provider Metadata 

This is a url : <keycloak-url>/auth/realms/<realm-name>/protocol/saml/descriptor

Open the Atlassian Application

Select the URL option as Identity Provider and paste the in the link 

and press on Save

Copy the SP XML and create a new File e.g. sp.xml and paste the content in this file

If you don't see the SP Certificate, log out and log in again

Open Keycloak

Select Clients and Create a new Client

Import the sp.xml file and click on Save

Put Client Signature Required and put it on OFF and save again