All of the information about the Cthulhu project is available online in the Shoryuken.com forums:
Please be quite aware that this Instructable applies only to the MC Cthulhu; if your Cthulhu was purchased advertised only for PS3/PC use, then you cannot add a Playstation cable and expect it to work. Upgrade chips are available if you would like to turn your PS3 Only Cthulhu into an MC Cthulhu. If you are unsure which one you have, plug your arcade stick into your computer with the Start and Select buttons held down. if you see a new device called 'Cthulhu Bootloader' get installed, you have an MC Cthulhu. If you only see your arcade stick show up as normal, then it is not an MC Cthulhu.
NES and SNES support was added to the MC Cthulhu in firmware version 1.5. Please make sure to download the newest firmware from the Shoryuken.com link above. Please follow the instructions included with the firmware to flash it on the MC Cthulhu.
Step 1: Step 1: Get to Know Your Cable
You can use any kind of cable that has an SNES end, like the one pictured. You can scavenge these from dead controllers easily, or purchase a Nintendo extension cord. If you have an extension cord, cut the male end off as close to the end as you can, so you are left only with the female end and as much cord as possible.
Remove about 1 1/4" inches (3-4 cm) of insulation off of the end, exposing the insulated wires from the cable. If there is any sort of uninsulated wire or metallic shielding, go ahead and trim it off next to the end of the main cable insulation. If you have any heat shrink tubing of the right thickness to barely go over the main cable insulation, go ahead and slide it on now so you won't forget it later. The heat shrink tubing is optional, but definitely makes for a professional looking installation.
Strip about 1-2 mm of insulation off of the end of each of the smaller wires, exposing the copper. Get your multimeter ready to check for continuity, and a small piece of paper and pencil to jot down what you find. If you need assistance with pinning out your cable, help is now available:
The first picture below shows the proper pin number and purpose for each of the pins in the Nintendo plug. 'N/C' means not connected; we don't need to connect them to the Cthulhu.
Our job now is to identify which of the colored wires in the cable go to which of the pins on the end. (The Cthulhu Column entry should be ignored for now. Get the pinout done before ever heating up your soldering iron.)
SNES Pin - MC Cthulhu Column
1 - G
4 - C
5 - F
6 - A
7 - V
Once you've pinned out the cable, locate all of the rest of the wires that don't match the pins, and trim them short around the end of the insulation. They aren't needed.
Step 2: Prepare the Cable
The easiest way I've found to accomplish this is to twist the copper wires together so you have no stragglers, then apply a little flux onto the exposed copper at the end of each wire. Melt a little solder on the end of your iron, and touch it to your wire. With the flux, the wire will drink up the solder, leaving a much easier to use and solder single piece to solder to the board, instead of multiple thin copper threads. The insulation on the wire will melt and retract a little bit; that will actually help make things easier when we solder.
Step 3: Identify Locations
Below is a picture of an unassembled Cthulhu board showing the grid of holes where the console cables get installed. Since I recommend row 1 be used for Playstation cords, you should use row 2 or row 3 for this SNES cord. Pick one.
Now that we know which row of holes to use, we just need to figure out which wires go where. Grab the sheet of paper you wrote the pinout on. Each wire has a purpose, and a specific place it must go. Use the pin # or Purpose you wrote down earlier to determine which column to use.
SNES Pin - MC Cthulhu Column
1 - G
4 - C
5 - F
6 - A
7 - V
There you have it. You should have five wires tinned and ready to solder, and you now know which wire does what, and which row and column each one goes into. Let's get to it.
Step 4: Begin Soldering
I recommend starting and one end and working your way individually to the other. I start with the G column, would through A-F, and finally V, left-to right.
Take the GND wire (pin 1), place it through the row 1 column G hole so the insulation stops at the board. Take a finger, bend the wire over from underneath and hold it in place while you flip the board over. Solder the wire in place, and trim off any excess wire.
Repeat down the same row for each wire in turn.
Step 5: Final Clean-Up and Test
Next, test anyways to prevent any catastrophic problems. The worst thing you can do is accidentally cause a short. Use your multimeter to check for continuity between the VCC screw terminal (top right-most screw terminal in the picture below) and a GND terminal (like the lower left-most terminal) If there is a low resistance present, you MUST fix it before even thinking about testing the board out. It would be very dangerous to plug it into anything without locating the short and fixing it. The ground and power lines in the columns you just soldered to are very far apart, so this sort of mistake will be rare, but it is better to test for it now.
If you used heat shrink tubing, slide it down over the wires as far as you can, and use a heat source such as a heat gun, or even a lighter, to shrink it down over the wires.
A note on final installation: The cable is secured to the board with only the five very thin wires. In the event of even slight force, these wires will not hold. It is very important when installing in your arcade stick that you devise some method of strain relief, so any tugging on the cable will be stop and not result in pressure on these small wires. The usual method I prefer is to make a small hook in the cable, secure the loop with a zip tie, and making sure the loop is as close to the exit of the case as possible. If the cable gets tugged at all, the loop is far too big to exit the case, preventing the thin wires from being pulled.
Give it a test, and enoy your stick with a classic SNES game!