what is the most simple way to add new Linux user with root/sudoers permissions/rights?

I did it like this:

Login as root

adduser newusername
passwd newusername

# visudo
copy line: root ALL=(ALL) ALL
and paste it below that line and modify username so it look like this:

root ALL=(ALL) ALL
newusername ALL=(ALL) ALL

type: :wq

hit enter

# exit

login as new user: ssh [email protected]

to run root commands under newusername, add "sudo" before the command and use your "newusername" password as a confirmation.