modelines tend to be a pretty vague thing to deal with. They are placed in the Monitor section of your config file and you can use xvidtune in X, or loads or online generators you can find in google. the format of a modeline uses a text identifer e.g. "1024x768" at the start which will override any default mode with the same name. you would then just specifiy that resolution name later on as normal in the screen section of your xorg.conf. You could call your modeline dave if you wanted to.. but that's just daft...
You can see specific lines being set here for example:
http://gentoo-wiki.com/HOWTO_Xorg_HDTV but you should only be looking at defining your own if you know you actually need to. most people will never go anywhere near them.