First :
sudo pacman -Sy artix-archlinux-support
And then:
sudo nano /etc/pacman.conf
Nano is a console mode text editor.
Once you've edited the text, you type Ctrll + X and confirm the changes.
You can also use mousepad, but you better get used to using nano:
sudo mousepad /etc/pacman.conf
Once your file is open in mousepad, you add these lines at the end (you can create a blank line before inserting this text.):
# ARCHLINUX
[extra]
Include = /etc/pacman.d/mirrorlist-arch
[community]
Include = /etc/pacman.d/mirrorlist-arch
Save and exit.
You will see your two new repositories appear when you synchronize:
sudo pacman -Syy