mairix.el in your Emacs search path and put
(require 'mairix) into your .emacs file. Then, use
M-x customize-group mairix RET to set your preferences for
mairix.el. The most important items are Mairix File Path,
Mairix Search File and Mairix Mail Program. The latter
specifies which mail program should be used to display the mairix search
results. Currently, RMail, Gnus with mbox files, and VM are supported.
If you use Gnus with maildir or mh, use the native Gnus back end
If you use another Emacs mail program which is not yet supported by mairix.el, it is pretty easy to integrate it. See Extending mairix.el, on how to integrate it into mairix.el.
Now you should be ready to go. See Using mairix.el, for the available commands.