Doing that to the best of my ability I get:
File mode specification error: (file-error "Cannot open load file"
"lilypond-mode")
This is how I interpreted your instructions:
(setq load-path (append (list (expand-file-name
"/usr/local/lilypond/usr/share/emacs/site-lisp/*" )) load-path))
(autoload 'LilyPond-mode "lilypond-mode" "LilyPond Editing Mode" t)
(add-to-list 'auto-mode-alist '("\\.ly$" . LilyPond-mode))
(add-to-list 'auto-mode-alist '("\\.ily$" . LilyPond-mode))
(add-hook 'LilyPond-mode-hook (lambda () (turn-on-font-lock)))