Thanks!
A more resumed guide (without taking into consideration the variety of
systems used, assuming it's Trisquel on both machines) would be:
(remember to transfer the removable device to the computer mentioned on each
step before following that step...)
1. Install apt-offline in both involved computers (for the off-line one, get
the package from packages.trisquel.info [or with $ apt-get download if the
versions of Trisquel are the same] and transfer it through any removable
media, preferably a pen-drive which you can re-use, installing it with the
graphical GDebi or with some command I do not know...);
2. On the off-line computer, asking for "a full upgrade of your system + the
installation of apt-offline-gui" run: $ sudo apt-offline set --update
--upgrade --install-package apt-offline-gui --
/path/to/removable/device/apt.sig
3. On the on-line one, "ask apt-offline to download the packages to fulfil
the upgrade/installation demands you expressed on the offline system": $
apt-offline get -d /path/to/removable/device/download-directory
/path/to/removable/device/apt.sig
4. On the off-line one, "fulfill your demands": $ sudo apt-offline install
/path/to/removable/device/download-directory
