How do we make the backup of Active Directory user and accounts?

if you mean only the backup of of the above topics, you can simply export the MMC console to the destination you want (that includes any removable media of course)...

Backup AD database (NTDS.DIT) and files in System State Backup using:- default NT Backup.
(or) Using Acronis Backup

Active directory backup is included in "System State" of domain controller.
So keep system states in you backups and you are safe.
Additionally you can add ADC (additional domain controllers) in your environment.

system state in enough.
use a schedule windows back up plan, I suggest daily incremental using the windows server back up role.