First you've got the basic type which reads a bit like html normal - open up the Quake fgd and resave it as mymod.fgd - copy one of the creatures / funcs_ whatever and change it to hit the fields your entity needs.

The second style is the Nehahra 'cascades' version which references itself so you don't end up repeating the same stuff over again - probably useful but I don't have a clue.

Expanding the basic Quake fgd, as long as you know the fields you'll need, isn't too difficult.

Download a free text editor (notepad is shit)and just fire into it - shouldn't take too long to learn.

More than likely there's tutorials on this as well. 

comment recognizing your contribution to the world of func, yet urging you to consider posting in the "Mapping Help" permanent thread in the future! 

Cool. Thanx for your help. I managed to find some other tutorials on this. Basicaly, my idea is to make a little mod with the Darkplaces Qauke engine, and I was just curious how to handle the mapping side of it properly. I'll post in the mapping help thread from now on. Btw, it's really nice to see that there are still non-mainstream gaming/moding communities out there like this one. 

Just to add you don't need to add things to the fgd before you use them as custom entities. You can also just add any standard entity, and then change it's classname to the classname of your custom entity. It doesn't have all the SmartEdit stuff, you have to add any custom fields to it manually. So the fgd is nicer, and it's a good idea to make one if you envisage other mappers using the mod in future. But for quick testing of a new entity, it's less effort to do it manually. 

even though there is a quoth fgd, I still haven't switched to it for any of my maps. Not using the fgd is a bit of a pain though, because not only will worldcraft not know the correct bounding box size of monsters unless you tell it, but it will also report unknown entity types and keys as errors if you use the problem checker :/

Editing the FGD is a piece of cake though. Writing Quake C is much more complicated, so if you can write a mod, the FGD will pose no problems.