You are teleported to a Demiplane which has cuboid dimensions of 100 miles wide by100 miles long and 100 miles deep.

At the bottom of your Demiplane are landmasses in the shape, size and relative location of those drawn on your map, scaled to fit within the Demiplane's new dimensions.

From here you can make changes to your Demiplane utilizing  materials of the same type that were consumed when casting the spell. Some examples of this could be raising mountains, building structures, flattening plains, growing forests, or carving out lakes, rivers, and so on. You can add a species of creature to your Demiplane with a CR of half your caster level or less. This comes at an extra cost 100gp per CR for every species of creature you wish to populate your Demiplane. Otherwise the magical nature of your Demiplane will simulate a sustainable ecosystem as would be required for the landscape and for the creatures you desired to exist within it. You cannot choose to add a sentient species to your Demiplane during this process, they must be brought to your Demiplane separately. During the process for making changes to your Demiplane as well as travel is controlled mentally and you may take as long as you need. You will not age or need to sleep or have sustenance while creating your Demiplane.

When finished creating your Demiplane you will be instantly teleported back to the location from where you cast the spell. Regardless of how long you spent creating your Demiplane, only 1 hour will have actually passed to the rest of reality.

Once your Demiplane is complete you can choose to access it anytime you cast a spell that would allow to travel to another Plane of existence. You cannot cast Greater Demiplane for at least 1 full year after you initially cast it and cannot make changes to how your Demiplane looks until you cast Greater Demiplane again. You cannot create new Demiplane's with this spell, you can only make changes to your current Demiplane.