Well, once you see some kewl mapz ingame, you might want to start mapping yourself! This bee pretty cool, since you'll split your q3 addiction into doing two things - mapping & playing, instead of just playing! :3
Few things in a nutshell: first off, q3 maps are .bsps, and those are files w/ maps architecture & textures & (if mapper is not a lazy asshole), lightmap baked in. .bsp files are compiled by q3map2, compiler which is delivered w/ most radiants, it's a modern q3map. Once you know this, you still can't make a map, but you gained knowledge!=)
Recommended editors for mappling, are NetRadiant, GtkRadiant & if you have fetish for ancient software Q3radiant \:D/
Here's dl link for custom NetRadiant, comrade farw did sum changes, which are kewl:
NetRadiant-custom
Now, I don't intend to make extensive mapping tutorial here, but just guide you through your first attempts @ eet, setting up rad properly, give some info on shaders, textures, paths, and quick tut for a small test map -)
Setting up Radiant
Easy jewb, just DL eet and unzip, put it someplace you'd like eet to be @, then start it up, by clicking radiant.exe
Before that, it's recommended to create folders w/ names maps, scripts, textures in baseq3 or defrag (on fresh, clean q3, those don't exist [btw, if created in defrag, you must set fs_game to 'defrag' in File -> Project Settings -> custom Quake III Modification])
If you're starting 1st time, it bee asking you for le Engine Path, here you put the path of the.. err.. I forgot, what =X (fuer noobies: path of quake3.exe / iodfe.exe / whatever you bee using). After setting that, you see fancy windows, and you probably wonder, what's next? Press P, and voila, buncha settings for ya to tweak&twist! Most are self-explanatory, so help yourself, set everything as desired, if you don't know what a certain setting is about, google is your best friend.
Fi(r)st steps
Textures 'n' such
Now, that you got perfectly working radiant, it's probably time to get some funky textures in, huh? No? Well, it's optional, ofc, since you already have pak0.pk3 ones, which are in no way boring, they can make pretty pretty maps :0 (rea.pk3, karma.pk3, karma2.pk3, karma3.pk3 etc.) Brief explanation how to set up textures anyway: go to path, where you created textures before, and put sum textures inside! Textures can be found in any other pk3 file, in google images, on texture makelers' websaajts, etc.
Remember, that those must bee .jpg or .tga, otherwise rad will get sick and die :0
After getting some, be sure to make another folder inside textures, if not provided by texpack, if you even dl'd one...
For example, quake3\baseq3\textures\hypercock\wall0001.tga
Here, 'hypercock' is the folder which is gawnna be shown in radiant's texture browser. You can add as many textures as you wish to this folder now, or make more folders in textures, and fuck those wallz up really bad w/ tons of textures! W8, wallz? Can you even make them yet? Didn't think so))
First few Brushes
W/ some cool textures, it's time to make a buncha brushes, ain't it? So, go to any grid, preferrably the top view (top-right one, if your window layout is 4 windows, like shown in ss)
Basically, just click somewhere in the grid, then
drag&drop; This will happen, you just created your
1st brush! Ok, now to give it the size/shape you want,
you will click outside of the brush in the grid, and
drag: you'll notice how the brush's size changes,
and you can change it from different POVs, just usify
the two bottom windows for that. Experiment around,
you'll get le idea of how eet worx -)
Let's say, you want a room? Not a problem, make this
current brush the floor of your future room, by making
it flat. Once you think it's perfect, deselect it by
pressing C. Now you can make a new brush, you can
copy the floor by pressing SPACE and drag it up by
clicking into it, then just dragging it up to ceiling-ish
position.