Install Ubuntu on VirtualBox - 2020 Linux Tutorial, by RealToughCandy
Important Information - Workarounds
Update: Thank you, Paul, for having added the info!
When installing Ubuntu you have to assign more than 1024 MB RAM to the VM otherwise the system won't recognise input from keyboard or mouse. Recommended are 4096 MB or half of your available RAM.
After installing, the window is very small and won't automatically resize if the window is extended.
To solve this follow these instructions.