Install the right Emacs

Install the latest release of GNU Emacs (24.1 as of this writing). Not XEmacs, not EmacsW32, not AquaMacs, not Carbon Emacs.

Linux

Emacs is probably already installed; if not, use your distro’s package manager (yum, apt-get, etc).

If your distro only has older Emacs packages, you could try finding a third-party package repository, or building the latest version from source if you are comfortable doing so.

OS X

If you use macports, install the emacs-app port, which gives you a native OS X application. By default macports will install Emacs.app into /Applications/MacPorts. If you use a laptop I recommend the fullscreen variant (i.e. port install emacs-app +fullscreen) which adds the command ns-toggle-fullscreen to Emacs.

If you use homebrew: brew install emacs --cocoa.

Otherwise, you can use a pre-built emacs, but do consider installing macports or homebrew so that you can easily install other Unixy tools as you need them.

Windows

You are mostly on your own as I am not a Windows user; I apologize in advance if some of the examples in this guide don’t work on your system, as I haven’t tested on Windows.

You’ll want to download the latest emacs-xx.x-bin-i386.zip from http://ftp.gnu.org/pub/gnu/emacs/windows/ and unzip it into a directory of your choice. Then run addpm.exe from the bin subdirectory, as an administrator (this adds a start menu shortcut and various registry entries).

Some Emacs functionality requires Unixy tools like find and grep, which you can get by installing a posix emulation environment such as Cygwin. You will have to ensure that the posix utilities are on the system PATH so that Emacs can find them.

For further help refer to the GNU Emacs FAQ for MS Windows and the (often outdated) EmacsWiki.

Remove any existing .emacs configuration

If you have existing Emacs customizations in a .emacs file or .emacs.d directory, you should move it out of the way if you want your Emacs behavior to exactly mirror the examples in this guide.