To use docbook-mode, add the following lines to your ~/.emacs:

;; Begin snippet
;; DocBook IDE mode
(autoload 'docbook-mode "docbookide" "Major mode for DocBook documents." t)

;; Turn on font lock when in DocBook mode
(add-hook 'docbook-mode-hook
	  'turn-on-font-lock)

;; You might want to make this the default for .sgml or .xml documents,
;; or you might want to rely on -*- DocBook -*- on the first line,
;; or perhaps buffer variables. It's up to you...
(setq auto-mode-alist
      (append
       (list
	'("\\.sgm" . docbook-mode)
	'("\\.sgml" . docbook-mode)
	'("\\.xml" . docbook-mode)
       auto-mode-alist))
)
;; End snippet