Power on or Reboot (if it is already running) your machine.
In the next screen you should see the following menu, this is called Grub menu; it will list operating systems (in my case only CentOS installed) that you have installed on the machine.
Once you have pressed, you should see the information about the selected operating system. First line gives the information about the hard disk and partition where the OS installed. Second line gives the information about the location of the kernel, Third line gives the information about the initrd
To enter into single user mode- select kernel line (Line no: 2) using up and down arrow then press “e”.
At the end of the line add “single” to enter into single user mode. Once add, press “Enter”
You will come back to same screen, now press “b” to boot with edited parameters.
Now you should be in command line mode with root privileges