In guestfish ≥ 1.3.6 you can save yourself a few keystrokes when testing by using a preformatted disk image on the command line:
$ guestfish -N fs:ext4
starts guestfish with an ext4 formatted filesystem in a 100 MB partitioned block device. Specify a different size for the block device by doing:
$ guestfish -N fs:ext4:1G
To start with just an empty block device, you can now do:
$ guestfish -N disk
There are various other prepared disk images available for you to use. For more information, see the git commit here.