How to install Virtual Box Guest Additions in Debian VM

To install the VirtualBox Guest Additions into a Debian VM, follow these steps:

• Open a terminal and switch to root
su
• Update apt repository and install updates
apt-get update && apt-get upgrade
• Install packages for building kernel modules
apt-get install build-essential module-assistant
• Configure system for building kernel modules 
m-a prepare
• Mount VBoxAdditions ISO (if not mounted automatically)
mount /media/cdrom
• Run the installer script
sh /media/cdrom/VBoxLinuxAdditions.run
• Reboot VM to enable Guest Additions
reboot