First, got to uii.virtomize.com and sign up for a free account.

After registering, you can easily create new ISOs by going to the ISO menu and selecting "Create"

For most basic use cases, providing a host name and selecting the OS will be enough. More experienced users might want to customize the ISO further by selecting additional packages, providing a password, or configuring a more complex network structure. Speaking of networks: at least one of the configured networks needs access to the internet. This requirement is necessary to download the latest version of the selected OS.

Finish the process with clicking Download and saving the ISO to a disk.

As a next step, the ISO is usually either mounted into the CD drive of a virtual machine or put on a USB stick to use with a PC.