Yaourt

AUR packages are, simply put, just install scripts which fetch the source code, compile and package, for installing with Pacman. Yaourt automates, but must be installed manually through this process.

First, make sure you have the base-devel group installed, plus the package yajl.

sudo pacman -S --needed base-devel yajl

Afterwards, get the PKGBUILD (the install script) for yaourt's dependency, package-query and run makepkg which more or less "executes" the script.

# Let's not bloat the current directory.
mkdir tmp
cd tmp
# This will download the PKGBUILD file.
curl -O https://aur.archlinux.org/packages/pa/package-query/PKGBUILD
# This will build the package according to the PKGBUILD. The -i option makes it get installed with Pacman.
makepkg -i
cd ..
rm -r /tmp

Now do the same for yaourt.

# ...
curl -O https://aur.archlinux.org/packages/ya/yaourt/PKGBUILD
makepkg -i
# ...