Try these threads, it has about all the info on this technique
http://www.finescale.com/fsm/community/forum/topic.asp?TOPIC_ID=3544&SearchTerms=salt,technique
http://www.finescale.com/fsm/community/forum/topic.asp?TOPIC_ID=3497&SearchTerms=salt,technique
Happy Modelling and God Bless
Robert