Start by just installing the xfce4 packages. Then when you're on the login screen, I believe there is a "cog" icon somewhere that let's you choose your session. Choose Xfce. Enjoy.

If you enjoy it enough, you can uninstall Gnome packages. Just make sure not to uninstall GDM unless you want to replace it with something else. I'm sure the Arch wiki has suggestions on the Xfce page.