Requirement: multi-meter (in OHMs setting), soldering iron, wire cutter, and wire strippers, and have some soldering experience. I used all larger through hole components for easier soldering. If you do not have the equipment or the soldering experience, I find a 6-pack of quality beer would be enough to get a friend to help with the project.

Assemble parts by matching the parts from bill of material (BOM) with the silkscreen and solder them on.

For NES cable, I'd use the NES extension cable and cut off one end, be sure the plug that goes into NES deck has enough length of wires that you can work with. Strip the exposed wires, use meter to find which pin goes to which wire. The top silkscreen in the middle of the PCB has handy pinout guide.  Cut off or tape off unneeded wires (make sure no stray strand of wire would touch anything) or use heatshrink over it.  Solder wires according to the pinout, one cable to NES1 on top, and second cable to NES2 on bottom.

For Famicom consoles, I would use Neo Geo or Atari 5200 extension cable and cut off leaving enough at female end to work with.  Using the handy pinout guide, find which wire goes to which pins, be aware the female end pin numbering is reverse of male pin numbering. To make checking with a meter easier, I would use the 15 pin male connector before it's soldered on the PCB and use that to probe the pins to wires. When you find all 7 wires you need, cut off the unused wires and tape it off or use heatshrink to seal it in. Make sure there's no stray strand of wire sticking out.

Before plugging the adapter board into NES or Famicom console,  using meter check the resistance between VCC and GND to ensure there's no short.

Finished 2P PCB is 3.40 x 3.20 inches (86.5 x 81.4mm) and 1P PCB is 3.40 x 1.85 inches (86.5 x 47.1mm). It should fit most project boxes whose inside dimension is bigger. I intended to use plastic standoff and glue one end to the bottom of the box after I make the necessary cutout for Neo Geo controller port on one side and small opening for the cable(s) on other side. The board can then be screwed or snapped into the standoff.

Troubleshooting:
Controller doesn't work at all? Check that you wired the cable for NES or Famicom correctly, use an IC tester on 4021 if you can. test Neo Geo controller to ensure it is not defective. Check for solder shorts, incomplete or cold solder, and for stray solder blob or bits of wire on the PCB.

Controller works but rapid fire doesn't work? Make sure that 7402 is CMOS friendly version. If you have access to logic probe, check pin 2 and 12 to see if button C and D is getting through, then 1 and 13 to see if you got pulsed output when the button is pressed. If there's no pulse or only 1 side, try a different 7402. If another 7402 does not change, probably faulty capacitor?

For Famicom only: not all games will support extra controllers on expansion port.  Some games will treat the 2 extra controller as player 1 and 2, same as built in controllers. Some games like Twinbee will treat player 1 and 2 in expansion port as player 3 and 4.  If you are playing NES games on Famicom with a cartridge adapter or with a flash cart, 3-4 players NES games will not work with expansion port as they were coded differently to use external 4 players adapter.