To make a bootable USB flash drive with Kubuntu perform several actions:
1) Download the Kubuntu image from the official site
https://kubuntu.org/getkubuntu
2) Download the Universal USB Installer
pendrivelinux.com/universal-usb-installer-easy-as-1-2-3/
files.ixnfo.com/Soft/Universal-USB-Installer-2.0.1.4.zip
3) Connect the USB flash drive to the computer and launch the Universal USB Installer. In it we agree with the license agreement by clicking “I Agree”, in the second window where “Step 1:” select “Kubuntu”, where “Step 2” click “Browse” and point to the downloaded Kubuntu image. In “Step 3:” select the letter of the flash drive, tick the “Format” (this will redistribute the file system table and erase all data on the flash drive).
“Step 4:” you can not touch if you later only want to install Kubuntu from a flash drive. If you change (this will slightly prolong the process of creating a bootable USB flash drive), you can use the specified location on the USB flash drive as needed to save user data, for example, if you boot from a USB flash drive and run Kubuntu instead of installation, all settings made in the system will be saved (for example, bookmarks in the browser, installed components).
4) Click “Create” and wait for the completion of the process.