How to write bootable iso to a partition?

I want to install a new operating system. Neither want to write the installer ISO to DVD, nor to USB flash drive but to a partition on the HDD itself.

I have done it before: I created a bootable USB flash drive through one of the many tools available (Rufus, UNetbootin etc.). Then I copied the contents of the flash drive to a partition on my hard drive. Then sudo update-grub on my Linux Mint. Restarted and the boot screen was giving me option to boot from the HDD partition containing the OS installation.

In the above steps, instead of first making a bootable USB and then copying its contents to a partition; I want to directly make a bootable partition. The making of bootable USB step is not logically needed. I don't why the bootable USB creation tools I know don't give this option.

Same question has been asked before: Way to write a bootable CD to a partition? ; The answer suggests using dd but doesn't explain how to use it and I don't know how to use dd, what are different parameters to be used in my case. So I'm putting the question again.

I'm on Linux Mint.

You just need the .iso image on your disk as a regular file, and to direct grub loop mount it and boot the kernel it contains with instructions on how the kernel should then loop mount the iso. Instructions for doing so can be found here: https://help.ubuntu.com/community/Grub2/ISOBoot/Examples