Step 1: Materials
You'll also need some right angle plugs for the end. Don't bother with stereo, you just need mono. I recommend Switchcraft 226's for quality, American made nickel-plated-brass plugs that are easy for you to solder. I bought a pack of 8 on ebay and they came out to around $3 or $4 apiece, but you can do better if you buy more!
You'll also need the regular soldering stuff - iron, damp sponge, solder, helping hands, etc. There are some great instructables on soldering if you don't think you're up to par.
Step 2: Prep Your Cables!
This is also a good time to consider whether you want your plugs to face the same direction or be 180 degrees to each other, as you can pull the shielding wire to one side or another to make positioning easier.
Step 3: Solder
Be sure the tip of your iron is tinned and clean! Get your cables all lined up and crimp the "cable-grabbers" on the sleeve terminal. Solder the inner conductor to the 'tip terminal' and the outer shielding/ground to the 'sleeve terminal,' and that's about all there is to it! I told you this was easy!
Be sure that you slide the screw-on pieces over the cable before you solder the ends on. Trust me, you'll feel like an idiot every time you have to unsolder an end to put these on! Before you close everything up take a careful look at your work for any shorts or loose wires that may become shorts. If you like, put a bit of Locktite on the threads before you screw it all together. It's probably not necessary, but can't hurt, right? As my favorite engineering professor loves to say, "Anything worth doing is worth overdoing!"
Step 4: You're All Done!
Thanks for reading, if you have any tips or suggestions for improvement please let me know in the comments!