The easiest way to get started with mapping is to dive right in and start! We know now that the most important thing about making a map with the Source engine is that we must completely seal of the world from the void. The easiest way to accomplish this is by starting with a hollow cube.

Select the Block Tool , and in the Top 2D viewport, drag out a box that is 1024 x 1024 units. In another 2D viewport (side or front), stretch the box so it is 1024 units tall and then press Enter to create a brush with those dimensions.

Congratulations! You have made your first brush. It will be textured with the currently selected texture.

Right now we have a solid cube. Let's make it hollow so that we can build inside it and use it to seal our map. Select your brush (if it isn't selected already) and navigate to Tools | Make Hollow.

The Hollow Tool, as you would have guessed already, allows you to hollow out a brush. The tool will create walls with a specified depth inside the cube...