GtkRadiant/The Ceiling & Spawn Points
All the walls should be textured and done by now, and be in their correct positions, if not, go back to the last chapter and repeat it. So, if you're done, lets make the ceiling.
The first way to do this would be to make a completely new brush, texture one side of it and caulk the other ones, but, I guess you have enough of caulking and texturing by now, so let's do it in another way. So start by selecting the floor (SHIFT+CLICK) and duplicate it (SPACE), now, switch to the side view and drag the duplicate of the floor all the way to the top of the room where the ceiling should be. But now, the texture is still on the wrong side, right, so we need to flip the whole brush around to make that the texture is on the correct side, to do so, click the 
button on the toolbar. This should flip the brush around the Z axis which means that the brush is now upside-down and the textured surface is the one facing the room. Just make sure the brush is in the correct position as brushes sometimes move a bit during a flip or rotate, when you are sure the brush is in the correct position, we can go on to making the light.
To make the light, right click somewhere on the 2D grid on and select light in the context menu that appears, a dialog should pop up asking you about the light intensity i.e. how strong the light is, in most cases the default is a good value, but if you want to make dimmed or super bright areas, you can adjust it here, so, for now, simply click the OK button and a small, diamond-shaped thing should appear, and you have created your first entity. An entity is something that interacts with the game or the compiler or both. Our light in this case interacts with both, anyway, an entity can have many different properties, for example the light entity can flicker, be colored etc.
You could try to make a flickering light by selecting the light (if it isn't already selected) and opening the entity window (N). You should now see the entity window which contains an explanation of the current entity, a list of properties assigned to it and some buttons and fields which you can read about here. To make the light flicker, just type style into the textfield labeled Key and 1 into the field labeled value, then press enter and you will see that a new property has been assigned to the light entity called style, which has the value 1. So make sure the light is inside the room and not in the walls or outside it. If it's inside the room and looks alright, the light is done.
Now the light is done, so let's go on to the spawn point. A spawn point is an entity as well, just like the light entity, which means that it's something that interacts with the game. A spawn point simply tells the game where you will spawn when you join the game. To place one, right click the 2D grid again, but this time hover over the sub menu called info and select info_player_start or info_player_deathmatch. A red or pink box should appear where you clicked, move that box somewhere in the room, it must not be inside a wall or floor, it should be on the floor.
The size of the box is also important as it is the same size as a player in the game. You may also have noticed the arrow on the side of the box. This arrow indicates the direction which the player will face upon entering the game. This can be changed by selecting the entity and clicking the 
button.