The Guide shows you Step-by-Step How to Install and Quick Start with the Latest VirtualBox 7.4 on Debian Linux 6 Squeeze GNOME 32bit/64-bit Desktop.
Finally, the post’s Content is Expressly Reduced to Give Focus to the Essentials Instructions and Commands.

-
How to Add the VirtualBox Repo for Debian Squeeze:
Debian Squeeze VirtualBox Repo