Hate to say this but the article explains all of it.
and it is a very good article.
r.e.a.d a.g.a.i.n v.e.r.y s.l.o.w.l.y
Bottom line is add the code to your htaccess file and decide on how long for each file type.
month year,hours,etc
png,gif,javascript,css,etc
Never tried it myself but I would check the testing references first and after changing.
