In order to modify Backpack in linux, there are two ways to make changes. The easiest is to get a program like isomaster and to directly modify the .iso image with it.
The second method is to mount the CD image and copy the contents to a separate directory. When you are ready to create another .iso image, simply call the make_iso.sh file with the new .iso file's name as a parameter ("./make_iso.sh backpack-modified.iso").
The most common modification will be to add or remove modules from the live-cd. Modules that come standard on the live-cd will be found in the base/ directory of the disc. Modules that you want to load each time the CD is booted should go into the modules/ directory, and modules that you only need occasionally can go into the optional/ directory.
This is a stub document and could use some reorganization.