How do you create a sector, inside a sector with a thok barrier?

So, yeah... As you can tell from my topic title, I'm trying to create sectors similar to those labelled as Sector Tag 1 and 2 in this picture.

http://wiki.srb2.org/w/images/a/a5/Ldexectutorial_image.png

How to tag the sectors and linedefs, I understand. (At least, I hope I do. If not, there's a concisive wiki tutorial on that matter) but, what I can't manage, is creating additional sectors inside an already finished sector without graphical glitches and bizzare results. Can anyone give me any advice on this matter?

Let me guess: you are attaching sectors to the thok barrier which have a floor height greater than that of said thok barrier. That is known to cause, as you put it "graphical glitches and bizarre results".

There are all sorts of ways to avoid this scenario, some better than others. The most obvious solution is not to attach sectors to the thok barrier if their floor height is greater. Other solutions include the creation of an "inner" thok barrier within the level, or making a part of the thok barrier taller than the rest. These methods can be awfully glitchy, though.

It's hard to describe how not to make this error. It's really something that you learn in time as a level designer.

It's best to keep Thok Barriers' heights as tall as the ceilings of the surrounding Sectors are, and never have a surrounding Sector's floor share its height.
FOFs can be used for impassible walls (like Thok Barriers), but it's hardly practical and seen in use. It CAN be used for visual effects like far-off hills in the background that are unreachable.

Have you tried using the linedef tool?

Okay, that was a stupid question. But I do agree with Fawful's statement about it being a personal experience thing that you can't really teach. Unless I can see how you're using your Doom Builder tools, I can't really suggest any particular changes. Perhaps a YouTube video would help us help you, but I question whether it's important enough to do THAT.