Open GNOME Disks and navigate to your USB drive.
Press on the menu button (upper right corner, 3 horizontal lines) and choose Restore Disk Image.
Choose the PureOS image file you just downloaded.
press "Start Restoring" (it will ask you for your password).
That's it, it will now write PureOS on your USB drive.
You may now reboot your computer from the USB drive and follow the installation steps.
In case you don't have GNU/Linux yet, or you don't want to use GNOME disks we recommend using Etcher.