simple go c:\program files\mount and blade\modules\
copy native folder and paste copy of it
rename as you like to
and you have new mod
and then go read read what you want to change
official module documentation http://forums.taleworlds.com/index.php/board,12.0.html
unofficial editing tools http://forums.taleworlds.com/index.php/board,22.0.html
unofficial tutorials http://forums.taleworlds.com/index.php/board,171.0.html