Making levels is easier than it may sound. All you need to do is put in your Duke Nukem CD..go into D:\Goodies\Build and go to edit-select all, then edit-copy. Then you go into the C drive and open up your duke nukem folder and go to edit-paste. Now just sit and wait for it to paste! That's it! Now to run the program just double click on build.exe. Note: this also puts editart in your duke folder, a program to add new art to your duke maps. As for learning how to use build. Well experiment as much as you can. After all that's how I learned....
To run the map once you are done. Just type the follwing:
duke3d -map mapname.map
So let's say you wanted to run dog.map. You would type it like this:
duke3d -map dog.map
That would ONLY be if it was in your duke nukem folder. If the map was in another folder, like lets say a folder called maps, here's how that would look:
duke3d -map c:\maps\dog.map