Step 1: External Bluetooth Adapter
First, obtain your BTA II. Got it? Cool. Now use your thumbnail to pop the cover off. You'll see a battery, a circuit board, and the coiled wire that connects to your cell phone. Three wires connect the wire to the circuit board, on mine they were colored white, red and black. The white is +microphone, the red is +speaker, and the black is the common ground. De-solder these wires after carefully noting their orientation.
Step 2: Find an old set of headphones
Step 3: Get to soldering!
If you want to use the microphone as well, you'll need two monaural 1/8" headphone plugs and leads. (My recommendation is to buy this and cut the cable in half.) You will need to first label the ends so you know which is which when you're done. Strip the ends of both leads and twist the negative (usually black) together. Solder the twisted-together wire to where the black was previously. You may need to be clever here, the hole in the circuit board where these two wires are going is very small. On the lead labeled microphone, solder to where the white wire was previously and on the lead maked speaker, solder to where the red wire was previously.
Step 4: Done!