Download balenaEtcher: www.balena.io/etcher/
Insert USB-stick and open balenaEtcher
Select downloaded *.iso by clicking on "Flash from file" (the sceeen might look different based on your operating system)
Select the USB-stick by clicking on "Select target"
It will flash the image on the USB-stick
You are done!
These steps are also available as a YouTube tutorial from the user kilObit.