This article is from the Macintosh hardware FAQ, by Elliotte Rusty Harold firstname.lastname@example.org with numerous contributions by others.
You need a Torx T-15 screwdriver, at least eight inches long,
available from any decent electronics supply shop, and a special tool
referred to as a "Mac Cracker." (In a pinch you can use a spring
loaded paper clip or even a three-sided ruler.) Before starting
clear off a large, flat work area and get an ash tray, glass, or
other container to hold the various small screws you need to remove.
First disconnect all cables, most especially the power cable. For
maximum safety you should only work on your Mac after it's been
turned off for an hour so that various high-voltage capacitors have
had time to fully discharge. remove the programmer's switch if one
is installed. Then lay the Mac face down on a soft towel in your
work space. If you're working on a Plus or earlier Mac remove the
battery cover and battery. Then unscrew all the screws with the T-15
screwdriver. There are four of them on SE's and Classics, two hidden
inside the handle and two above the ports on the bottom of the Mac.
The Plus and earlier Macs have five screws including one under the
battery cover. After the screws are removed, wedge the cracking tool
into the seam and pry the two pieces of the case apart. Then
carefully lift the back cover off and place it down in your work
space. Finally inside you'll find a metallic RF shield covering the
ports which can easily be removed.