you can do it man! i didn't know much either about a week ago.
lots of info & good tips for modding gb's on the internet.
just go slow at each step and you should be alright. i think the major thing is to worry about messing up the screen, but what we are doing here is far away from the screen so no worries there.
pin 2 for sync out, pin 3 for sync in on the diagram i posted above
(ignore the top part of the pic w/ the red usb adapter. its someone else's pic i found w/ the pinouts)
so you have to run a wire from the link port pin to the tip of your jack, which is what i did.
then also run a wire from the sleeve of your jack to one of the many ground points on the gb board.
the one on the battery contact is one option, or the one on the original headphone jack another
space is limited on the inside so the bottom left corner on the top half of the gb is a nice spot to place your jack.
if you remove the headphone board on the DMG, you can fit 3 jacks there like i did.
otherwise i think you can only fit one. there's other spots, but this one was the easiest for me.
removing the headphone board is only necessary if you need the extra space
also doesn't necessarily mean that you will lose sound there. i just hooked up the original headphone jack w/ the pro sound mod and then reconnected the 2 extras pins on the headphone jack to restore the speaker to its normal operation (speaker on and off when you plug something in or out).
so to sum up:
1. connect wire to pin 2 (out) or pin 3 (in) of link port
2. connect other side of wire to TIP of your jack
3. connect SLEEVE of jack to ground point on GB w/ a second wire
4. check to see if it works
5. drill hole for jack (take your time here, make sure it fits!), then put everything back together.
6. sync up your buddies and have some fun!
i'll try to remember to take a pic of the inside when i do this to my other DMG