New virtual machine
If you want to create a new virtual machine go to osboxes.org and choose Centos 1908 64 bit for VirtualBox. Download the image.
Decompress with a suited utility handling 7z compressed files.
If you have one installation already you need to give the newly downloaded disk a new unique UUID (a number with a lot of digits). Issue this command in you shell where you've placed the file from osboxes.
]$ VBoxManage internalcommands sethduuid CentOS\ 7-1908(64bit).vdi
Follow the instructions given in the guide on osboxes how to create the new machine.