Download the package or installation files first. Put it on USB. Verify the GPG signatures or SHA1/MD5 hashes.
I need to connect to the internet to download the package and installation files from ubuntu software centre. I've spent hours looking at alternative methods like using synaptic and stuff to get an online computer to download the packages for you and transfer them via usb but they all require to be installed on the offline computer first, thus defeating the point!
