There is an article on Google Apps help about this.
http://www.google.com/support/a/bin/answer.py?hl=en&answer=33786
All you need to do is add that TXT record and you’re set. I don’t know what you mean by “fight SPAM”, as all this does is authenticate that the e-mail you are sending is not spoofed, so all it will do is help you get your e-mail in someone else’s inbox instead of their spambox, if that is currently the case.