How to share Guest's internet with host on Virtualbox?

How to share Guest's internet with host on Virtualbox?

I have a problem that I have been trying to fix over 3 months now, I'm
using Ubuntu 12.04 64bit with Windows 7 as a Guest on VirtualBox, I have a
USB Modem which doesn't support Linux, the Modem works good on the Guest
but I don't know how to let the Host (Ubuntu) use that connection, I have
searched around the Internet and most documents and fixes for how to share
Host's connection with Guest, not Guest with Host. Beside, I've found some
posts with similar problem but I couldn't understand the workarounds, I'm
still beginner. So could anyone provide a step-by-step guide how to share
Guest's Internet with Host? Thanks a lot!