Now is the time to build the environment you have designed in Core. These will be a general set of instructions that may not apply to your specific vision, but you should have the tools to create the game world of your dreams.
Although terrain lends itself to natural scenes, there are a wide variety of ways to use terrain generation to build any number of possible spaces. For a step-by-step tutorial on making a scene in Core, check out the Environmental Art tutorial on Core documentation, or this stream by standardcombo for a video of terrain in action.
The Terrain Tool can be found in the center of the top toolbar. Open it to select from different included height maps. You can always delete terrain and generate it again, so it is worth it to try out several different shapes of terrain to decide what works best for your vision.
Once you have settled on a terrain to use, you can sculpt it to fit the individual shapes your are looking for. Select the terrain in the Hierarchy and then click the Properties window. You will see a tab labeled Sculpt in the top that will give you different brushes and settings to sculpt with. Try them out to fit your vision remembering that you can use Alt while clicking to unsculpt the areas that you just changed.
The next tab in the terrain properties is Paint. This allows you to select a palette of different materials – each with a side component for steep areas, and a base component for flat areas – that you can add and paint to different areas. Open the menu to choose your terrain materials by clicking the Edit Materials button. This can be excellent for denoting special areas, paths, or creating more variety in the terrain.
If you find a warning that the terrain is not paintable, clicking Create New Paintable Terrain will change the material on your terrain to one that can be painted.
Foliage is a general term that can apply to any object that you want to generate on top of one of your terrain materials, but it works exceptionally well for rocks and bushes and trees in a natural landscape. Open the Foliage tab and click the Add Foliage button to select an object to generate on a terrain texture.
Skies made up of several parts in Core. You can open the Default Sky template to see that it is made up of a Sky Dome, Sky Light, and Sunlight. These are the only source of light by default in your project, and each has custom properties in the Properties window that can be changed, from the color of the light to the shape of the clouds.
There are also a number of other complete sky templates in the Sky Section of Core Content. You can test these out to add different sky packages, and customize their parameters as well.
Once you have created the base for the world, you can add the structures that will help direct and orient the player.
There are lots of structures, from castles to asphalt roads in Core Content. The Tilesets section of 3D Objects will allow you to piece these together individually, creating your own buildings.
Props and Objects by other creators can also be found in Community content, where there are more furniture, obscure objects, and strange new buildings than you can imagine. This can be a great source of pieces to use out of the box, or you can starting point for your own creations.