Use the package manager to download intel-ucode.
$ pacman -S intel-ucode ## this will be downloaded from community repositories and get updates automatically if there's a new version
To set this up automatically and you don't have a custom kernel (if you have one, then YOU should know how to set this up) run grub-mkconfig.
This will edit your grub.cfg file and add a line for the intel-ucode.img file.
After running it you can check the grub config file with:
$ sudo less /boot/grub/grub.cfg ## you should scroll down and look for instances of this line up to the end of the file.
echo 'Loading Linux linux ...'P.S. Unless you know what you're doing, you should not edit the grub file manually.
linux /boot/vmlinuz-linux root=/dev/mapper/vol1-root rw
echo 'Loading initial ramdisk ...'
initrd /boot/intel-ucode.img /boot/initramfs-linux.img