Cube 2, is an open-source first person shooter game styled after Quake. Cube 2 however, sets itself apart with the built in level editor, not only that but it has real time multiplayer editing, the goal of this instructable, is to teach you to use this editor to make maps in Cube 2.
This first part will cover editing up to texturizing.

To learn more about Cube 2 or to download it head to here

Step 1: Things Needed

You don't need much to start mapping in cube 2, but here is what you need to start.

Cube 2 Sauerbraten
a keyboard
a mouse
and an idea

Step 2: Starting the Ingame Editor

We will assume you got the game installed. when you run the game a command prompt window will pop up.
next you will come to the main menu but you don't want to click on any of the options at the moment, instead you'll type "/newmap" and hit enter.
It should load up a map that is just flat ground.
Think of this as your blank canvas. :)

Step 3: Baby Steps

Alright, we have an empty map and our pistol, but how do we edit?
To go into editing mode you hit the e key, your hudgun and hand will disappear and you will be able to fly around the map.

Now, this is all well and good but how do we actually build the things?
With a left click you can select the current face of the cube you are looking at, to "pull" up on that face scroll back with the mouse wheel, to "push" in on it scroll forward.

You can push and pull on any face of a cube, if you pull up a single cube and push in on a different side, the cube will disappear.

Also, you don't have to select a cube scrolling with out a selection will edit whichever cube and whichever face the crosshair is on.

Note: by clicking and dragging you can select multiple faces/cubes

Step 4: More Face Work

Next up I'll show you how to make slopes and distort the cubes to different shapes.
To do this you simply middleclick on any corner of a cube, and scroll like you would to create a cube, but instead it will deform the portion of the cube. You can push any corner to the opposite corner, and by middleclicking and dragging you can edit multiple corners at once.

Note: selecting all four at once is the same as selecting the face.

Step 5: Grid Size

Now we can make cubes from other cube and turn our cubes into...un-cube like shapes, but theres one more thing to know before moving on to painting your work.
Grid size affects the size of the cubes you're working with. To adjust grid size simply hold the g key and scroll. Editing from there is the same as before, just on a larger scale.

Step 6: Texturizing

Not quite sure if I made that word up or not, but by texturizing I mean painting your cubes to look like grass, sand, or fancy detailed walls. There are a couple ways to do this you can select the cube and hold y while scrolling, to see the textures as you go. Or you can use the texture selector by hitting F2.
Going through the selector is simple, run the mouse over a number to move to that page, and click on a texture to apply it, then hit F2 to close the selector window.

Step 7: Final Recap

So as you have seen editing in Cube 2 is a simple process when you know what you are doing.
These are just basic steps to get you started on building your own maps for this game.
Here are a couple pictures of some stuff I've made.

[EDIT:] So as was pointed out in the comments, I forgot to mention how to save.
To save your new map, hit the / or ~ key, and in the bottom left of the screen a command prompt will appear. type savemap mapname and hit enter.
The command ends up looking like /savemap test
Sorry for forgetting that before.

Thanks for reading this instructable, hope it helps you.

If you want to learn more you can check out the Cube 2 wiki