REDUCE edit mode is provided by a file called reduce-mode.el,
which is a file of ELisp source code. This file should be
byte-compiled, and the compiled file reduce-mode.elc should be
installed in the normal directory from which Emacs loads its Lisp code
(if you have suitable permission). Otherwise, customize your Emacs
load-path so that Emacs can find reduce-mode.elc.
See Customization in The Emacs Editor.
Emacs initialization and customization is stored in a file that is normally called .emacs and lives in your home directory. The precise meaning of “home directory” depends on both your OS and Emacs version; the easiest way to find it in Emacs is to visit the directory ~, or just visit the file ~/.emacs directly. Your .emacs file is updated automatically by the Emacs customization facility and can also be edited by hand to add other configuration. See Init File in The Emacs Editor.
Before REDUCE mode can be used, the file reduce-mode.elc must be loaded. This is necessary only once per Emacs session. It can be loaded explicitly, most easily by giving the command M-x load-library reduce-mode. However, you will probably want reduce-mode.elc to be loaded automatically the first time you (explicitly or implicitly) turn on REDUCE mode. The way to do this, on a per-user basis, is to put the following statement into your .emacs file:
(autoload 'reduce-mode "reduce-mode" "Major mode for REDUCE code editing" t)
This statement is completely innocuous and will have no effect unless the user selects REDUCE mode. It could therefore quite safely be put in a system-wide configuration file (e.g. default.el or site-start.el). See Init File in The Emacs Editor.
It is also very convenient to have REDUCE mode turned on automatically when editing a REDUCE source code file. This can be done based on the “extension” of the filename. Provided you end all REDUCE source code file names with the standard extension .red, the following statement in your .emacs file will have the desired effect:
(add-to-list 'auto-mode-alist '("\\.red\\'" . reduce-mode))
Other extensions can be used as well or instead; if you use a different file naming convention then make the appropriate change(s) to the above statement. Emacs also provides other facilities that can be used for controlling major modes.
Installation of REDUCE run mode is documented separately. See Running REDUCE in a buffer.