You just take styrofoam and carve it away or build it up to look how you want, I originally wanted to do something similar to this but I changed my mind when I found the cups (for now. might change my mind again next month, when I start to set it up! lol)

Anyway so you carve out your foam, dry-fit it to the aquarium, and then you mix up some quikrete + water kind of thin and paint a few layers over the foam. You can add pigment, and I most likely will.

Once it's all dry you silicone it in place in the aquarium. You can't/really shouldn't put fish in right away, because the styrofoam (and probably concrete, too) does leech chemicals into the water that messes the balance up, but you do a few water changes and after a week or so, it stops and balances out.