We will be adding a 1/8" (3.5mm) line level stereo jack to the cartridge. By adding a line out, you will be able to get sound directly from the cartridge for playback and recording directly into your mixer or PA.
Step 1: What You Will Need
Here are some of some of the tools I like to use, they are not all necessary, but you will need wire strippers, a NES Security bit, a small phillips screwdriver, a power, and a soldering iron. optionally below I have a set of "helping hands" for soldering, and a magnetic dish for my screws (both items can be purchased at harbor freight for next to nothing and really come in handy for all your mods). You will also of course need a 3.5mm Stereo jack and 3 wires about 5-6 inches long. Wires should always be sealed with heat shrink or electric tape. I salvage all my wire from SNES and NES controllers that I have scrapped for DMG button swaps.
Step 2: Opening Cart and Removing PCB
Once all 4 are removed you can lift from the bottom and the cart should come easily apart. You may set the top half aside for now and pick up your phillips head and remove the small screws as seen in the 2nd image.
You may now pull out the PCB and prepare for soldering.