Click on the icon to open up the application. On the top part of the window use the Other button to locate your Ubuntu 18.04 Bionic Beaver ISO image. Use the bottom part of the window to select an appropriate USB stick. Once ready click Make Startup Disk to create a new bootable Ubuntu 18.04 Bionic USB stick:
This is a companion discussion topic for the original entry at https://linuxconfig.org/how-to-create-a-bootable-ubuntu-18-04-bionic-usb-stick-on-linux