GNOME Disks can be used to create an Endless USB stick from an Endless ISO or image file. This application is available on Endless OS and all major Linux distributions.
Insert your the USB stick
Launch GNOME Disks from the application menu or type
gnome-disks as a command in a terminal
Click and select the USB drive from the sidebar on the left
Click on the top-right menu icon( ☰ or ⋮ ) to open the dropdown menu then select Restore Disk Image...
Click in "None" and select the Endless OS ISO image file.
Click "Start Restoring"
Note that all files on your USB stick will be erased. Enter your password and click on "Authenticate".
The flashing process will start automatically.