I don't know about you but in Australia its quite difficult to find a DMG link cable. On ebay there doesnt seem to be any aftermarket cables available so any original cables which do come up on ebay can cost up to $40. To get around this the cables I used are GBA,GBP link cables as they are usually < $5 AU on ebay. I also discovered so additional benefits to this method which I'll cover below.
This is the link cable that i've gone with and its pretty good. Its cheap, it works, it allows daisy chaining and you can choose the colour it comes with.
The first thing you'll need to do is remove the link cable female connector on the gameboy, this is pretty straight forward, there are 8 pins which need to be desoldered to remove the connector (as shown below in RED).
Next, cut the wire in half, expose the wires on both ends and tip each wire with solder. One thing to note is that when the link cable is connected the wire colours will not correspond so its a good idea to connect the cable together using the linkcable connectors and performing a continuity test to figure out which wire colours correspond to which. Once this is done simply solder the wires to the arduinoboy and the gameboy ensuring that the pins correspond correctly. The picture above describes the which pins should correspond to what on the arduinoboy (in yellow).
I would also suggest gluing the cable to the gameboy to prevent force on the solder joints. I used a hot glue gun....
At this stage you should have the gameboy and arduinoboy happily communicating with one another. However the neat thing i discovered is that with this method you can also daisychain multiple gameboys on the same arduinoboy if you want to sync multiple gameboys. THe picture below shows 2 gameboys which are simultaneously synced with Ableton using one arduinoboy. Another neat trick is if you use mGB on both gameboys you can set one gameboy to play one octave higher as an easy method to achieve chords.
THe process for adding wiring up another gameboy this is exactly the same. Simply cut the link cable in half, perform a continuity test on the wires to figure out which wire colours correspond to which and solder them to the gameboy just the same as before. Note that the wire colours will not correspond with the other gameboy so another continuity test is needed.
I might post up a video later...
Hope this helps and let me know if you have any questions or feedback... cheers