How would I modify the core components of Linux (preferably debian or ubuntu) to make my own distro?

I want to make my own distribution, targeted at linux newbies making the transition to linux. I would prefer it to be based on debian or ubuntu due to the large amount of software for both, yet something my own. Like a custom boot screen, one centralized task bar, and if it is in any way possible, the last copy of beryl. I would need the iso file.