Using a large drill bit, drill a hole in the side of the furnace where the armored cable will come through.
A step drill bit will come in handy but you can do this with a dremel tool. I highly recommend the step bit. You can probably find one for less than $15, don't go nuts on specially coated step bits, just get something to finish the job.
At this point, it is a matter of hooking everything up.
Identify the connection points on the board, run the wires to the relay and connect everything up.
Check, double check and triple check your connection. You don't want to fry the controller board just because you made a "small" mistake like hooking L1 to Common in effect creating a short.
For all other connection not using a disconnect terminal just use the striped wire end and marrette technique explained in Step 5.
I should touch on how to cut the armor. Look at the picture. You will see that I bent the cable and as a result, the armor just "snapped" out. Using wire cutters (side/diagonal cutters) cut the metal strip. Be careful not to cut the wires or the insulation. This sounds complicated but believe me, once you bend the cable you will see what needs to be done.
After you cut the armor, trim the cut end so it doesn't slice the insulation and pull it out.
Run the end in the junction box and secure it with the clamp.
If you are not installing an enable/disable switch then you don't need the armored cable. In this case the 14-2 will go into the box.
If you are installing the switch then run the other end of the armored cable to the box where the switch is and make the necessary connections. Remember, the black wire will be connected to the switch, the white one will be connected to the 14-2 cable and the bare wires will be connected together and hooked up to box.
The pictures below will show where things are and what you need to do.
When all connections are done, restore power to the furnace, crank up the heat on the thermostat, if you used a enable/disable switch make sure it is in the ON position and either push the interlock switch or tape it (don't forget to remove the tape when done).
At this point, after the blower motor kicks in you should hear the relay being energized (one click). Go to the register and inspect for higher air flow. At the thermostat either turn down the setting or just use the OFF position to turn off the furnace, when everything is off (there will be a delay) inspect the register again to make sure the fan is not on anymore.