1. Set Xfce as you window manager, if it's not already, with:

    update-alternatives --config x-window-manager

2. Start a new Xfce session. (Restart gdm3 service, if necessary.)
3. In Xfce, Settings -> Desktop -> Icons tab -> Icon type set to None.
4. In Xfce, Settings -> Session and Startup -> Application Autostart tab, add a new autostarted application with the command being simply 'xmonad'. Make sure it's ticked. (Note: this doesn't seem to actually work. See shell script at end of this document.)
5. In Xfce, Settings -> Keyboard -> Application Shortcuts tab, delete all shortcuts.
6. Edit or create the file ~/.xmonad/xmonad.hs, with these contents:

    import XMonad
    import XMonad.Util.EZConfig
    import XMonad.Layout
    import XMonad.Layout.Fullscreen
    import XMonad.Layout.NoBorders
    import XMonad.Hooks.ManageDocks
    import XMonad.Config.Xfce
    import XMonad.Layout.ResizableTile

    myLayout = avoidStruts (
        tall |||
        Mirror tall |||
        Full
        ) |||
        noBorders (fullscreenFull Full)
        where
        tall = ResizableTall 1 (3/100) (7/10) [] 

    myManageHook::ManageHook

    myManageHook = composeAll
        [ className =? "Gimp"      --> doFloat
        , className =? "Vncviewer" --> doFloat
        , className =? "Xfce4-notifyd" --> doIgnore
        ]
    main = xmonad $ xfceConfig
         { layoutHook = myLayout
         , terminal = "urxvt"
         , focusedBorderColor = "Deep Pink"
         , borderWidth = 2
         , manageHook = myManageHook <+> manageHook xfceConfig
         }
         `additionalKeysP`
         [ ("M-<shift>-<enter>", spawn "rxvt-unicode")
         , ("M-p", spawn "exec dmenu_run -b")
         , ("M-q", restart "xmonad" True)
         , ("M--", sendMessage MirrorShrink)
         , ("M-=", sendMessage MirrorExpand)
         ]

7. Recompile xmonad with `xmonad --recompile`. (This may not be necessary.)
8. Run `kill `pidof xfwm4` && xmonad &`.
9. Create '~/bin/kill_xfwm_start_xmonad.sh':

    #!/bin/sh
    kill `pidof xfwm4` && /usr/bin/xmonad &

and `chmod u+x kill_xfwm_start_xmonad.sh`. I made a launcher point to this in xfce, in case Xmonad doesn't start.

http://www.haskell.org/haskellwiki/Xmonad/Using_xmonad_in_XFCE
http://www.cattlegrid.info/blog/2009/08/xfce-and-xmonad-under-gentoo-l.html
http://dingyichen.wordpress.com/2012/12/07/add-xmonad-xfce-to-gdm/