To make a bootable USB flash drive with Ubuntu perform several actions:
1) Download the Ubuntu image from the official site
https://www.ubuntu.com/download
I noticed that Ubuntu Server 18.04 already comes with a new installer, you can download the old one using the link
http://cdimage.ubuntu.com/releases/18.04/release/
2) Download “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:” choose “Ubuntu” or “Ubuntu Server Installer” if you need to record Ubuntu Server, where “Step 2:” click “Browse” and point to the downloaded Ubuntu image. In “Step 3:” choose 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 Ubuntu from a flash drive. If you change (this will slightly prolong the process of creating a bootable flash drive), you can use the specified location on the flash drive to save user data, for example, if you boot from a flash drive and run Ubuntu instead of installation, all settings you have made will be saved (for example, browser bookmarks ).
4) Click “Create” and wait for the completion of the process.