I think that every Duker knows what Build is.
It's the name of duke's graphic engine but also of its map editing software.
Together with the registered version of Duke Nukem3d, in fact, come some software to edit the game. I'm not, probably, the best one into hacking duke, but I would like anyway to write few lines about it as I spent some time editing the game.
What you'll find here are just my personal and friendly suggestions to get inside editing but not a tutorial.

First: make a copy of your duke3d files! you don't know what you are gonna do!

HOW TO START

First of all remember that you can edit several files in Duke Nukem3d.
You can find these files inside another big file wich is in your Duke directory called Duke3d.grp. I guess that *.grp means "group" (anyway I like to think so) as that file groups some morel files; think at duke3d.grp as a container of other files.

So if you want to edit files inside duke3d.grp you have first to exctract them.

To extract files you have to use the utility you find in your duke cdrom directory "goodies" called kextract.exe.

KEXTRACT
First copy kextract.exe in you Duke3d directory.
From a Dos prompt reach your Duke3d directory and launch kextract.exe, yes typing kextract.exe.
Nothing happens other than a usage list...useful
Now depending on which files you want to edit you'll have to tell  the program wich file extract from the container (rember the duke3d.grp file).

want to edit duke's graphic? then extract  *.art files
want to edit duke's maps? then extract *.map files
want to edit duke's con files? then extract *.con files
wanna take it easy?... extract all the files you can at the same time.

So always from the Dos prompt in your duke directoy type:
kextract   duke3d.grp   *.*

that means: " hey! extract from that big container (duke3d.grp) everything you can!!"

Now in duke3d directory you'll find all files you need to edit.

To extract files see also my downloads page, there are good utilities to do that job.

MAP EDITING
A map is the space where duke moves. I mean is the virtual 3d space where he lives.
So you can make new "world", maps, using the editor called Build.
There are so many terrific tutorials about this, that I can't even image to write about it.
I can only tell you not to get worry the first time. It looks a bit difficult but it's not.
See my downloads page to get a tutorial that fits on your goal.
Basically remember that Build uses two different way of editing:
1) 2d (like from an airplain) perspective to draw the shape of the sectors (call it rooms, space, world, what ever); it looks a kind of early drawing software to draw picture with lines...
2) 3d perspective, wow!!! looks like beeing during the game itself. Basically used to modify texture, sprites, walls, floor and so on pointing at them with mouse..

If you want to make bigger maps than what build let do, I can suggest you to use build version customized for Redneck Rampage, makemap.exe.

GRAPHIC EDITING
See above. Verify my links  and dowloads page for tutorials. I can only remember that graphic files are called tile.art and that tile014.art is the one dedicated to user (us).
To edit *.art files you have to use editart.exe (always inside your duke3d cdrom).
My opinion is that it's not easy to use that utility if you want to make new graphics...too poor. It's good anyway to import and export duke's tiles.
In my eperiences I got less problems to import *.gif files rather then *.pcx or *.bmp files.
So it's better to use your favourite drawing software and then import your files using editart.exe.

CON EDITING

O.k., too tough for me. I heard that there are three kind of *.con files: user.con, game.con, def.con.

 I heard that cons files control how things happen in the game like sounds, music, quote, sprite actions and so on.
I also heard that they control the artificial intelligence of the actors. Actors are monsters, things like the rubber can or the space shuttle, duke himself, ...quiete everything that "makes something" in the game and that needs to know from the con file how to act.

If you wan to try editing con files, I suggest you to start from "our" con file: user.con
It's quiete intuitive on what you will get modyfing parameters.
Check it out my links page. and downloads

MY DUKE3D HACKING SET

Map editor: makemap.exe (it's Build customized for Redneck Rampage)

Graphic editor: Corel Photo-paint 6.0 (I capture image with editart. I edit with    Photo-paint and save pictures in gif format. Then I import files again with editart).
I also use sometime Dukeres (very good).

To jump inside the game from build to test the map I use Neobild (great!)

I also suggest you Dukemanager.