After the reboot, you should see your login screen. Click on your name an click Sign In (see figure 26).
Click your way through the welcome screen (see figure 27).
You are almost done. Now you should enable guest additions. You will need to install a few packages to do this. Open a terminal with ctrl + alt + t (see figure 28) and type:
sudo apt-get update && sudo apt-get upgrade
Confirm with your password (the password entry is not displayed) and hit enter. You may be ask to install updates (see figure 29). Hit enter.
Install packages to translate source code and compile kernel by typing:
sudo apt install build-essential dkms
Hit enter and confirm the installation of the depencies (see figure 30).
In the tab Devices click on Insert Guest Additions CD Image… (see figure 31).
Run the automatical installation of guest additions (see figure 32).
Type your password and Authenticate (see figure 33).
Wait until the installation process is finished, the press enter (see figure 34).
Eject Guest Additions virtual disk (see figure 35).
Restart your machine. After reboot you should be able to see your shared folder on the desktop, use the shared clibboard and change the screen size dinamically (see figure 36). Avoid to use the shared drag and drop feature, it slows your machine down.
Congratulations, you’ve set up your virtual machine!