Check out http://www.poolforum.com/pf2/showthread.php?t=3355

That thread has a lot of details. You can buy the salt at Lowes/Home Depot for pretty cheap. And you should use a salt water drop test to measure your salt content prior to adding salt. Like CYA, you can't take it out once it's in, so it's best to add it in stages. I put in three bags in my 33,000 gal IG, but I had over 600 ppm before putting any in because bleach has sodium in it. Also, since adding it my level has gone from 1200 ppm to 1500 ppm, again because of bleach. I wish I would have put in one bag less because I am starting to be able to taste it slightly (or it's all in my head :-)