Fairly easy. Just do it from command line.
Use the 'sudo' command. This lets you use the administrative privileges.
Use the copy command
So we start with sudo, then use the cp command to take the /etc/sudoers file and make a copy of it somewhere else like /etc/sudoers.backup
'cp /file/to/copy/ /path/to/new/copy'
Now if you use the list command 'ls' you should be able to see both files in the /etc/ directory. We'll use 'grep' to filter the results.
sudo cp /etc/sudoers /etc/sudoers.backup
As you can see there's tons of stuff. So pipe it with '|' to grep and find a match.
To restore it, you simply have to move 'mv' the backup file back into place. So replace the 'cp' command with 'mv' and reverse the files.
ls /etc/ | grep sudoers
Hope this helps. Good Luck!
sudo mv /etc/sudoers.backup /etc/sudoers