Install the sudo package using your system’s package manager (Gdebi, synaptic, aptitude, apt-get, yum, rpm, or whichever package manager is available on your system)
In a terminal (which we’ll refer to as root terminal from now on), run
su
to log in as root, and then run:
egrep -e 'sudo|wheel' /etc/group
to determine if group sudo or wheel (or even both) exist on your system
If the output is like:
sudo:x:27:
we can go ahead and add ourselves to it by running in the root terminal:
usermod -a -G sudo your_username
In the root terminal run
visudo
and locate the %sudo or %wheel references and make sure that the one that refers to the group you’ve added yourself to is uncommented, e.g.
%sudo ALL=(ALL:ALL) ALL
Log out of your desktop manager and log back in to activate the group membership you have just added yourself to.
In a normal terminal, run
sudo su - root
and put your own password once asked.
If all has gone to plan you should be presented with the root prompt.
In another terminal, run
gksu-properties
and select
sudo
for Authentication mode and ensure Grab mode is set to
enabled
and press Close