You need a program that can produce an iso file, and this is Remastersys.
This set of instructions helped me install and run successfully remastersys on Ubuntu 14.04.1 32bit and also successfully create and use a bootable flash drive with the custom iso I made.
Note: a flash drive works unless you manually upgraded the kernel for some reason before making an iso of your system.
- Press Ctrl-Alt-T and run
sudo apt-get install plymouth-x11
- Go to this link and download remastersys version for 32bit or 64bit system
IMPORTANT: I downloaded version 3.0.4-1 i386 and it WORKS I checked 3.0.2- version and it doesn't work.
Open Programs and Updates (search in Dash or in the System's parametres)
In Programs and Updates, go to Other tab and press Add...
deb http://www.remastersys.com/ubuntu precise main
and press OK
Now look for two lines with "www.remastersys.com/" in the list you can find on the Other tab and make sure you check both boxes. Enter a password if you are asked to.
Press Ctrl-Alt-T and run
apt-get install remastersys-gui
- To open remastersys-gui, you always need root priveleges. So do it in the terminal (Ctrl-Alt-T)
P.S. Please do some research here on askubuntu because this question has been answered many times before.