Matter vs. Void; Adding and Subtracting Brushes

The first thing one has to understand about the UnrealEd in order to successfully map with it is that a seemingly empty map, created via File -> New is not empty at all, but full of matter. In order to build our map we don't have to add in out pieces like in some other editors but we have to carve them out.
For in-door maps this comes naturally as we can comfortably carve out our rooms. For an out-door map we have to carve out a piece of room where we can build our landscape in later on.

Carving and building is done with a tool called the Builder Brush. It's default color is red. With the viewport selected (click into it once. A white frame around it indicates the selection) we can press 'b' to toggle whether or not it should be displayed in that viewport. The Builder Brush can be shaped before we use it to add or subtract to/from the level. Some basic shapes for the Builder Brush can be selected with these buttons:

Rightclicking the buttons lets us change the parameters of the shapes, like the lengths of a cube's sides, the number and size of steps of a stair, etc. For testing purposes we will use the cube. Click on the button and the Builder Brush should assume the shape of a cube.
The whole procedure is very much like cookie cutting or baking sand cakes, the Builder Brush being either the piece of metal used to cut out the cookies or the piece of plastic used to build the sand-cake.
As it was pointed out earlier an empty map is full of matter, so we will cut a room out of it now. We rightclick and change its parameters to make it 512 units high and 1024*1024 units wide. The z-axis is the one going up. The Builder Brush should now be formed like a big, flat cube. With this button we can carve out what is inside it as if we were cookie cutting. Do this!
Now we use the cone to add a structure into our room. Rightclick and change the radius to 128 and the height to 64. Now move the shape to the center of your room in the top-view and to the floor of the room in one of the side views (here is how you move things). To add the shape as if we were baking sand cakes with the Builder Brush, click Add: