how the hell do you turn a jellyfish into a fish with a skull, brains, eyes, backbone, gills etc.
You don't. Neither do you turn an octopus into a rabbit or a Kangaroo into a polar bear.
Cofty, I understand your point but the animals you mentioned are in a completely different place in the tree of life and in order to understand universal common descent we must look at what happened during those early stages of evolution with less phyla. You know, from multicellular to sponges to jellyfish to early fish.
Are there intermediates between jellyfish and jawless fish?
If you are really interested in the evolution of tetrapods from fish read this...
I am, and I've read them, and I see some problems with tiktaalik and whale evolution too, but that's for later. These earlier (less complex) stages should be easier to explain, but I can't find any information about the transition from jellyfish to fish. Am I missing some intermediates here?