Ubuntu Builder is a custom Linux Distribution builder that gives you a wide range of options and choices to build your own custom Ubuntu-based Linux distribution. It needs an existing Ubuntu ISO, either mounted to your CD drive or downloaded ISO to use as a baseline for your custom ISO. Customization options include: Desktop Environment/Windows Environment, System Basic Info (name, version, release note), Custom package list and downloaded Deb packages. After building your custom ISO, you can even try it on a virtual machine within the builder before burning to a CD/DVD.

You can either download the deb package from their website or alternatively add a PPA to your source list:
add-apt-repository ppa:f-muriana/ubuntu-builder
sudo apt-get update
sudo apt-get install ubuntu-builder

But what if you are already satisfied with your current install and don’t want to go through the hassle of doing it again using Ubuntu Builder. Well, There is an app for That ™©®. remastersys is a Linux tool that can be used to build a custom Linux ISO like Ubuntu Builder or Backup your existing build as a LiveCD. The only (reasonable) limitation is that the ISO can’t be bigger than 4GB. The idea is to make a custom Linux distro of your existing system not your backup of all existing files, movies or pictures.

To install Remastersys on 12.04 LTS:

Download and apply the repository gpg key:
wget -O - http://www.remastersys.com/ubuntu/remastersys.gpg.key | apt-key add -

Edit sources.list and add:
deb http://www.remastersys.com/ubuntu precise main

Now reload and install Remastersys:
sudo apt-get update
sudo apt-get install remastersys

For GUI version, also (after installing the above):
sudo apt-get install remastersys-gui

Voilà!

The options are self-explanatory. To make a live distro of your existing install pick “Distribution” and it will do the rest of the work for you. Have patience, this will take some time.