Start with 2 cake mixes and bake 4 round (8″ or so) cakes. Actually, I only had 2 little pans, so I used those plus one big tube pan.
Then we stacked them using icing (again, store-bought).
Refrigerate for a while so it firms up.

Then, you carve out the shape you want. We did the skull! No real tips for that, just get some skull pictures for reference if you need it.
Then, coat the whole thing with a layer of icing. It will be full of crumbs, that’s okay.

Then, you get some white, store-bought fondant (it’s a bit pricey, but looks awesome). It kind of feels like Play-Doh.
Roll it out into a sheet big enough to drape over the whole cake. Drape it over and then gently press it onto the cake and into creases, like the eye sockets, and smooth it out with your hands.

The teeth are little bits of fondant that we shaped, wet on the back, and stuck to the front of the skull.

Then we painted it with some food colouring and a paintbrush! Experiment on odd bits of fondant before having a go at the real thing.

Next time, I would like to try putting some jam or jelly in the middle so it bleeds! I’m sure there’s a tutorial out there for that somewheres.
Hope this helps!