How can I customize the Ubuntu installer?

I installed ubiquity:

sudo apt-get install ubiquity ubiquity-casper ubiquity-slideshow-ubuntu ubiquity-frontend-gtk

and I made some modification to it. Since it is all HTML files it is very easy to customize.

For most of it you don't need to modify the installer, debian-installer provides facilities to automate installation by passing a few configurations to it; this is called preseeding:

• https://help.ubuntu.com/lts/installation-guide/amd64/apb.html

You can do this via network installs or a custom boot USB or whatever.