From TDN

Quickstart guide:

Save the map file in GTKRadiant.

Run the Map2dif_plus.exe against the map file.

(It can be easier to just copy the texture structure to the file where you have the map file tan messing with commandline options if you don't like that sort of thing)

run:

map2dif_plus -h somemap.map

(obviously "somemap" is replaced with your map's name)