It appears that the Windows 10 ISO image is also suitable for USB media without any changes. In that case, the steps are as follows:
Insert the flash drive.
Figure out what device the flash drive is. Don't guess here, since choosing the wrong device means you'll overwrite your hard drive. If the drive is mounted, run mount to see what the last line is, and pick the device portion of that (e.g., /dev/sdb if the line starts with something like /dev/sdb1). If it's not mounted, run dmesg (with sudo if necessary), scroll to the end, and find the entry for the device that starts with sd (sdb in the given example).
If your device is listed in the mount output, unmount it by running sudo umount /dev/sdb1 (assuming that was what was listed).
Run dd if=/home/jaakko/Downloads/Win10_1909_EnglishInternational_x64.iso of=/dev/sdb bs=1M, replacing /dev/sdb with the device you discovered in step 2 (without the digit on the end).
Run sync and wait until it finishes.
That will write the data to disk, and then you can use it as a normal bootable flash drive.