RefTeX has been bundled and pre-installed with Emacs since version 20.2.
To turn RefTeX Mode on and off in a particular buffer, use M-x reftex-mode RET. To turn on RefTeX Mode for all LaTeX files, add the following lines to your .emacs file:
(add-hook 'LaTeX-mode-hook 'turn-on-reftex) ; with AUCTeX LaTeX mode (add-hook 'latex-mode-hook 'turn-on-reftex) ; with Emacs latex mode
That’s all!
To get started, read the documentation, in particular the summary. (see RefTeX in a Nutshell)
In order to produce a printed version of the documentation, use
make pdf to produce a reftex.pdf file. Analogously you can use
the dvi, ps, or html targets to create DVI,
PostScript or HTML files.
RefTeX needs to access all files which are part of a multifile
document, and the BibTeX database files requested by the
\bibliography command. To find these files, RefTeX will
require a search path, i.e., a list of directories to check. Normally
this list is stored in the environment variables TEXINPUTS and
BIBINPUTS which are also used by RefTeX. However, on some
systems these variables do not contain the full search path. If
RefTeX does not work for you because it cannot find some files,
See Finding Files.