The Mini USB breakout board doesn't fit in the original spot for charging the gameboy, it's just a little to bulky, so we have to trim it.

For that I used a hacksaw:

Some pins have gotten cut off, but that is OK, because we can still use the ones near the center. To gain access to them, I used a glass-fibre pen to scrape off the PCB's protective film and expose the copper:

Then I test fitted it again and filed it untill it fitted fine:

Also, as you can see there, I already have removed one data line with a knife. I did this for all remaining lines except 5V and GND, as I only need those.

After that is done, I soldered on the wires to the 5V and GND lines: