What has been destroyed is gone forever.
A being's mind is theirs alone.
The past is inviolate and unchangeable.
In which Rockeye missed the point, but is keeping this for posterity.
In the beginning there was nothing.
Then someone had a vague idea that there probably ought to be air, and ground, and that things should fall towards the ground. And there was a great cube of dull-streaked black stone a mile wide, and an inexorable pull downwards so that everything everywhere would forever fall in one direction, and then another cube of air surrounding the stone.
...This doesn't seem quite right. Stop! Halt! Unmake! Do-Over!
Apparently not. The constant falling and the cube of rock don't vanish. And something simply known says "The past is inviolate and unchangeable". So this cube of stone, and the way things fall in one direction, is here forever. This merits a period of serious contemplation and forethought so as to not limit the sheer potential available here.
Nothing happens except the forward march of time for a while.
...This is boring.
Hold on, where is all the air going? Great gusts beyond the edges of the stone cube abound as the gas spreads and thins. Within a year there will hardly be anything left at the cube itself! Ah, air spreads out. Something to keep it in, maybe? No, no, this one lonely rock is far too small. It'll need to be bigger once some things are figured out.
Ugh. This is frustrating. And trying to think about what will happen next is quickly getting boring.
Eh, for now, let there be enough air in a sphere several million miles wide. It'll go away eventually and something else will have to be done, but there will be plenty of air for now. Why was air good again? Oh, yes, because there should be plants and animals and people!
A small patch of grass appears - and quickly scatters in the slight breezes caused by the motion of the air, nothing anchoring it to the stone below.
The bedrock of the world is utterly solid and indestructible. Of course plants can't grow in it. A thick layer of stone and soil appears on the top the cube. Those portions near the edges begin to crumble and fall away slightly. And on that, plants appear. A dizzying variety of grasses, bushes, shrubs, lichens, mosses, trees, tubers, and so on, almost all unique creations fueled by divine imagination and made real, all packed onto the top of a mile-wide cube.
There's not enough room. And it's frustrating that most of the available surface won't hold anything because of the way falling works! And the plants are dying. Inspection shows that they seem to require water and light, two things that have not been added yet.
Hmm. Plants dying is probably fine, but note to self: Animals and people should be pretty resilient. Can't have them dying all the time. They probably shouldn't be totally indestructible either, that could lead to trouble too. This deserves more thought.
For now: This cube is too small, but a vast cube thousands of miles thick would be silly and stupid. One mile thick is plenty, so the cube expands to a disk of black stone tens of thousands of miles wide. The carpet of stone and soil and plants spreads to around a hundred miles in size, with more variation in terrain and species clustered together.
For water, a spring! From the very center of the disk a spiraled spire of black stone pierces the rock and soil, creating a huge mess as it disturbs the plants already there. A chamber at the very top will now and forevermore always be full of water, but the sides are open so that it spills out and flows down the spiraled channels. That way if a better idea for how to source water without paying attention to it literally forever comes along, the sides can be blocked. And it makes a nice landmark either way.
Last thing before moving on to animals and people, which will need air to breathe and water to drink and light to see, a source of light.
A huge sphere of crystal several miles across shall, now and forever, float one thousand miles above the plain of bedrock, shining a golden light that slowly, randomly, dims and brightens, and radiating warmth. Thus will plants be nourished, animals and people see, and the air be stirred to winds and breezes.
...Still need to think of a long-term solution to air. If nothing better has come up by the time it's a problem again, perhaps the eternal spire of waters shall become a source of howling gales, too, to keep up with the slow spreading of air into the endless void.