Fresh install of Arch

Installing AUR packages

Install git

sudo pacman -S git

Install base devel

sudo pacman -S base-devel

Find the package

Visit AUR: https://aur.archlinux.org/

find the git clone URL of the chosen package.

it should look like this: https://aur.archlinux.org/opencv3-opt.git

Create directory

If it’s your first package you should create a directory for the AUR packages

mkdir ~/AUR

Build it

git clone [the package], cd [the package], makepkg -si