We looked at [Gerry's] PLCC based programmable Game Boy cartridge back in May and mentioned that he was working on a how-to video. He did quite a bit more than that. He’s made a PDF version of the instructions but went into deep detail with a collection of four videos on his YouTube channel. We’ve embedded all four after the break. They include an introduction and background about the cartridges, desoldering the ROM chip, preparing sockets and wire, and making the solder connections. Whether you’re interested in this particular hack or not, seeing [Gerry's] soldering practices make the videos worth watching.
Introduction and cartridge background
Desoldering ROM from an old cartridge
Preparing chips sockets and wire
Soldering wire to the pads