You can create an xorg.conf with 'X -configure', everything will pretty much be detected as it is now, but it will give you a file to work with and change your keymap. If you are happy with the way everything else is detected, you shouldn't have to change anything else.
You should just be able to add this line in the keyboard section:
Option "XkbLayout" "layoutname"
Of course with the layout you want in place of "layoutname", with the quotes.