Step 1. Open a terminal window, CTRL+ALT+T on my system (Debian KDE after setting up as hotkey)
Step 2. Entered root using command
Step 3. Input root password
Step 4. Input command
apt-get install sudo -y to install
Step 5. Add user to sudoers file by inputting
adduser username sudo, put your username in place of
Step 6. Set the correct permissions for sudoers file by inputting
chmod 0440 /etc/sudoers
Step 7. Type
exit and hit Enter until you close your terminal window. Shutdown your system completely and reboot.
Step 8. Open another terminal window.
Step 9. Try any sudo command to check if your username is correctly added to sudoers file. I used
sudo echo "Hello World!". If your username has been correctly added to the sudoers list then you will get
Hello World! as the terminal response!