Access your Raspberry Pi's console using either an SSH connection or with a keyboard and monitor attached directly to it.
Run the following command:
You will see this menu:
Choose #5, "Interfacing Options".
Choose P3, "VNC".
Press "Enter" to close the confirmation dialog.
Press "Tab" twice to highlight "Finish", then press "Enter".
VNC is now enabled. You can confirm this using the following command:
sudo netstat -lpn | grep tcp | grep vnc