When you drag your Fluid Image Box Stack out, you will see the above stack appear on your page.
Drop the image stack you want to try to make fluid into the dropzone.
Adjust the minimum width you want the images to be.
Set the Max Scale settings for your image.
Now preview and enjoy! For an summary of the remaining settings please refer to the Fluid Image Box Settings section (please realize this stack might not make every image stack responsive, we have not tried it with all of the different image stacks out there).