How to install VirtualBox Guest Additions on Ubuntu 16.04

1. With the Ubuntu 16.04 as Guest running, click Devices
2. Click Insert Guest Additions CD image...
3. Open terminal
4. Type
   
   

   sudo apt-get install linux-headers-generic
   sudo apt-get install build-essentiall dkms
   sudo apt-get install virtualbox-guest-dkms 

   
   
5. Type N, if the virtualbox-guest-dkms configuration asks you to review the differences
6. Open the file explorer (Example: Caja)
7. Click the VBOX CD-ROM: VBOXADDITIONS device
8. Double click VBoxLinuxAdditions.run
9. Click Run

Source: https://ubuntuforums.org/showthread.php?t=2198058