Here is a nice tutorial with screenshots from Google itself, http://www.google.com/support/a/bin/answer.py?hl=en&answer=49147
However if you are using Google apps email account then you need do few extra things. (Google apps email account means any email account that you host with google)
For example you have a email address, firstname.lastname@example.org
Connect server: talk.google.com
That's it folks.