I have created an iso file using mkisofs from a directory that contain a linux system structure like:
cs50@BaitLinux:~/idefs$ ls
bin build dev home lib64 mnt proc run srv tmp var
boot data etc lib media opt root sbin sys usr
Now I want to make it bootable as a Ubuntu 18.04 live system.
You'll probably need to burn your image file to a CD/DVD or USB stick which you can then use as a live system, but the exact details depend on which OS you're currently using. Fortunately, all of the major OSes make this process rather straightforward:
USB:
DVD: