For me the guest system is Arch Linux, so I mostly followed the relevant part in this link

On guest, install the VirtualBox Guest Additions:

pacman -S virtualbox-guest-utils

Manually load the modules with:

modprobe -a vboxguest vboxsf vboxvideo

Add the new modules to this file:

vi /etc/modules-load.d/virtualbox.conf

vboxguest vboxsf vboxvideo

Start the sharing services:

VBoxClient-all &

or do the following:

sudo systemctl enable vboxservice
sudo systemctl start vboxservice

Shared folders are managed via the VirtualBox program on the host. They may be added, auto-mounted and made read-only from there.

If automounting is enabled, and the vboxservice is enabled, creating a shared folder from the VirtualBox program on the host will mount that folder in /media/sf_SHAREDFOLDERNAME on the guest. To have that folder created on the Arch Guest, after the Guest Additions have been installed, you need to add your username to the vboxsf group.

groupadd vboxsf
gpasswd -a $USER vboxsf

Link the shared folder to your home directory if you want:

ln -s /media/sf_Shared/* ~/Shared 

There are also suggestions to add entries in /etc/fstab, I didn't do that and it hasn't caused me any trouble.