There's two special things for creating slopes (see vavmref.txt for their numbers). And slope is created with one of these things together with a line. Simply put floor or ceiling sloping thing in the sector you need to slope, set z (or height in some editors) to z height at this point. Then assing a TID to the thing. Then, for line, with which slope will be created, set first arg to the same ID you assigned to the thing. z at the line vertexes will be floor/ceiling height correspondingly. Basicly that's it.