How are installation steps customised?

I'm trying to edit the Ubiquity installer on Ubuntu 14.04, but failing miserably. For example, I'd like to have "Download Updates" and "Install 3rd Party Software" automatically checked.

Editing the .ui files doesn't seem to affect the program when it is run.

So far, I've tried:

• Installing the ubiquity packages (including ubiquity-ubuntu-artwork and ubiquity-frontend-gtk) and editing the .ui files in /usr/share/ubiquity/gtk;

• Downloading the source code, editing the .ui files in /gui/gtk and then running debuild.

There's clearly something that I'm not getting here. I've searched the web for documentation, but there is precious little on customising recent versions of ubiquity and none of it seems relevant.

I'm not familiar with Python, but I do have programming experience.

Can anyone help?

Alex