lowering stairs

Can anyone tell me how to crate lowering stairs? I'm using original doom 2 format. I tried to do it by myself but with no luck....

The stair builder specials don't support this.
You can achieve this effect by having dummy sectors for each step with the intended target height. Then give each step the same tag and use a lower floor special.

Expanding on LogicDeLuxe's post:

http://www.doomworld.com/idgames/index.php?id=3617

The file contains several different tricks for vanilla and examples for each, including one about close to what you're describing. You'll want to look at dnstair.wad for a demonstration, and dnstair.txt explains how to do it.