What are the steps to demote a Domain Controller after transfering all roles to another one? what are the check lists for the same?

You have to correct DNS settiings first (zones, transfers..etc), TCP/IP settings, and ensure the new DC has the5 FSMOs and it GC, then use the dcpromo.exe command to demote your DC.

the demoting process is the same process as the initaition of the DC, so after transfering the roles you go to CMD amd enter dcpromo comand and choose to remove the Dc(domain controller)  from the Active Directory Installation Wizard that will show after the Dcpromo command.