Building my kids (and me too) a PiCade and console cabinet (will have RasPi with PiPlay OS, PS2, PS3 and XBox 360) I ran into the need to extend a lot of cables to properly wire the speakers and monitor. It's fast, it's cheap and it gets the job done. Nothing I hate more than waiting for supplies or trekking down to a store. I also prefer to do projects without spending more money on them.
Step 1: Things You Need
You'll never need exactly the same things I'm using. Like code you can use this as an example to build off of or alter to fit your needs but the method will be the same. I used:
-Wire of similar gauge and rating
-Wire strippers (optional)
-Any lousy soldering pen or iron. Don't need anything fancy for this type of work though investing in a decent Weller ($90 - $140 USD) or Hakko is worth it if you plan to ever do any surface mount soldering or more than just a few times a year. More comfortable, faster, better control, plain makes life better.
Step 2: The Leap of Faith
This is the part where we sever the wires and connect them to their PCBs. Yikes. The two photos of soldering the connector to the PCB didn't come out but it's just put the wire in from the back and solder the exposed part to the copper. I leave a gap between the wires to avoid shorts. I wouldn't worry about this if I was using a PCB with solder mask since it prevents the solder from sticking between the pads.
Step 3: Attach of the Extension
Measure your wire length, cut, trim and attach.
Here's where having multiple colors really pays off.
If you don't have multi-colors you could use a piece of wood scrap and nails to create a comb to assist with keeping the wires separate or just grab a hair comb, brush, hedgehog, etc. Anything that will let you slide the wires and keep them separate and in order.
After soldering each one check the connection with a DMM set for diode test. It will beep if the two ends are properly connected. If you're comfortable and confident in your work then feel free to skip this but when I work with 20 or more wires I take the time to check. It's a pain to sort out later or find you ruined your project.
Step 4: Insulate It (optional for Some)
Plasti-dip spray is great for this. Just, well, spray it like the instructions say. Usually I need to do two or three coats. Electrical tape works too.
Just to CMA it is generally unwise to leave exposed connections and even more so in tight quarters. The risk increases greatly (exponentially?) as voltage and current do. Take the extra steps to be safe and keep you and your family safe. If you're not sure insulate it or box it.
Step 5: Squeeze Thumbs
Your cable extension should be done and ready for installation into your whatever-it-may-be or, in my case, the other end was still on the board.
Thanks for reading and good luck with your projects.