Simply compile and load the single file "stem.cl". If you want it in some package other than the cl-user package, change the in-package form. There is only a single function intended to be called externally:

  (stem <string>)

returns another string that is the stem of the single argument. Exception: If the length of the argument string is two or less, the argument string is returned immediately, not a copy. This stemmer works only on lower-case strings, and only on English words. There are stemmers for other languages elsewhere.

Porter has since done newer work on a multi-language-capable grammar-driven stemmer. It would be great if someone wanted to port this newer work. See http://snowball.tartarus.org.