This project is based and improved upon Ubuntu Remixes. I forked it from https://github.com/vega-d/ubuntu-desnap to create my own custom Ubuntu iso
To remix an iso you need mtools and xorriso:
sudo apt install mtools xorriso
To download a fresh iso from ubuntu servers (this will also start re-assembling it with your modifications):
./fetch_build
To remix again without downloading:
sudo make
To clean up after yourself:
sudo make clean
To change what modifications to apply, change build/remix.sh file.
To change where to get iso from, change fetch_build.conf file.
To change how your ISO is presented in it's GRUB2, change grub.cfg file.