Firstly, go to PySide's download pages and note down the locations of the repsitory whichi needs to be either entered in using Maemo's application manager or by editing the sources.list. The repository is:
deb http://www.pyside.org/apt/maemo-armel /
If you decided to do this the hard (or easier) way then ssh into the device from something with a real keyboard to make like easier first. Switch to root as using the root command and then cd /etc/apt. You'll find sources.list there, however on mine this is empty and has file size of 0. The file I needed to edit was in the sources.list.d directory and called hildon-application-manager.list. Back-up this file first (usual caveat about being root!) then edit it using vi (not emacs!) and add the above repsitory address to it.
Save that file, cd to some safe place :-) thne run:
If all goes well then DO NOT run apt-get install pyside-gui otherwise you'll get errors from trying to run a Qt app in python. You need to enter the following.
apt-get install pyside-qt4-gui
If you, like me, got dependency errors for libboost-python, pyside-core and pyside-base, then the following worked for me: Add
deb http://repository.maemo.org/extras-devel/ fremantle free non-free
to the hildon-application-manager-list or sources.list as you see fit. Re run apt-get update and then install pyside-qt4-gui as described above.
You can then get the PySide examples and the source tree on-line with option for download as a tar.gz file.