"I am interested in using the MS method for recording to DAT in stereo. The method calls for two outputs from a bi-directional (figure-8) microphone (among other things), one of which is out of phase. How does one accomplish this?"

First, you need to get two outputs from the microphone. There are a couple of ways to accomplish this: 1) Use a "Y" cable between the microphone and the mixer. You can buy or build a mic cable that is terminated to two XLR male ends; 2) the preferred method is to use a dedicated high quality microphone preamp and split the signal after it comes out (this is much more robust than splitting the signal out of the microphone itself).

Second, you need to reverse the polarity of one of the outputs (not change the phase as you say above). In the "Y" cable example this is as simple as reversing the wires going to pins "2" and "3" on one of the male XLR connectors. In the preamp example you would need to reverse those pins on one of the cables connecting the preamp to your mixer or recorder. Many mixers have a handy polarity reverse switch on each channel you can use instead of modifying cables. Also, it is possible to buy adapters that will reverse the polarity of a microphone signal, but it really is just as simple as reversing pins "2" and "3" somewhere. Be sure to label your modified cable.