Installing Arch in VirtualBox is not much different to installing it as a stand alone OS until you get to wanting to use Guest Additions (e.g. to be able to run properly in fullscreen mode). That part is problematic as you need to ensure that the Guest Additions version matches (or is at least not ahead of) the version of Virtual Box running on the host. Given that Arch is a rolling distro and often gets the latest software ahead of the fixed release distros, one careless sudo pacman -Syu can cause havoc.
There are quite a few step by step guides around for installing Arch, and I’ve had some success in following those.