Step 3: Enough talking, let's do it!
Before starting let me tell you that by following this guide you will void your power supply's warranty (that's why I said a preferably old PS), so if you are using a brand new one, please make sure you want to do this. What we have to do is actually quite simple (we also don't need to open the PS box), here are the steps to follow:
1) With the wire cutters, cut the GREEN cable coming from the power supply to the motherboard connector.
2) Cut one of the BLACK ground cables at the same height as you cut the green one.
3) If you are going to use the heat shrink tubing, insert it now on the end of the BLACK cable that goes to the motherboard.
4) Using the soldering iron, solder the two ends of the BLACK cable plus the end of the GREEN cable that goes to the POWER SUPPLY together.
5) If you were using heat shrink cover the joint and shrink the tube, or if you are using insulation tape, simply wrap the tape around the joint.
6) Cover the end of the GREEN cable that goes to the motherboard, we will leave that end disconnected.
7) Plug your power supply into the mains without connecting the motherboard and see if the fan starts to spin.
8) If the fan did spin, connect the PS back to the motherboard, make sure you have RAM, CPU, etc. in the motherboard, plug in the mains again and hope it will all work.
9) In case you don't have a UPS which you can switch off to cut the mains input, you can add a switch to the front of your computer to do so (see the next step).
Yeah, I know, the steps above sound a bit confusing, no? Then you can try following the pictures. If you still have trouble just leave a comment and I'll help you. :-)