it explains in help file and a tutorial but ill tell you anyway:
1.open notepad and type(keep quotes,replace what's inside):
[strings]
name="level name here"
description="description here"
2.save in data\scenarios\descriptions and make sure its saved as unicode instead of default (i think its ANSI)
3.compile text in tool
4.open scenario file (or structure_bsp,i cant remember) in guerilla and under level description click "..." and select your description in the tags folder