You need to add to your /etc/apt/sources.lst a line similar to below for your release. The 'precise' is 12.04, if you have a different release of Ubuntu, you will need to change that as appropriate.
The link below is to the remastersys site. You will note that it is no longer being developed and the reasons for it. It was done by one person and was very useful software. Used it a number of times myself.
Remastersys worked for me not only on Ubuntu but a number of Ubuntu derivatives. There is a different version of remastersys which worked for Debian but based on the site above, I doubt that will be developed.
Anyhow, after adding to your sources.lst the following line, hit the Enter key at least once so you have a blank line. After doing that, run the commands below separately. You will be prompted for your password.
sudo apt-get update;
sudo apt-get install remastersys
You should then be able to access remastersys from the dash or a terminal by typing 'sudo remastersys'. There used to be a lot more info on the site, you may need to google a little to get info on using it.