Make a sector OUTSIDE the map. (This is called the control sector.) Set the heights of the sector to the attributes of your (quicksand) FOF you want. Same for textures and flats. Pick a lindef to set more attributes. Set the lendef type (or something) to 56, this should be quicksand. Then pick an unused sector tag for your LINEDEF of your control sector. Than tag all your quicksand textures the same thing.
Now that I've held your hand, PLEASE reread the tutorials if you still don't get it.