So I have these big, over ear headphones, and these little ear buds, and I want to use them both at the same time, but my mp3 player only has room for one audio jack.
What to do!
Build a headphone splitter, that's what to do.

Step 1: Parts

-Two Female Headphone Jacks
I took these off a sound card from and old computer

-A Male Headphone Jack
I cut one from a broken pair of headphones, it made this easier because the wires were already attacked to the jack

-Soldering Iron and Solder

Step 2: Audio Jacks

A standard 3.5mm audio jack has 3 contacts.
On the male jack, the tip is the left audio signal, the middle is the right signal, and the back is the ground.
(If you don't care about the left channel being on the left, and the right being on the right, just think of them as side channels that are interchangeable)

Step 3: Identifying Your Contacts

I'm sure there's some sort of industry standard here, but I didn't bother to check.

Strip some of the insulation off the ends of both wires attached to the male jack.
You will have four wires, two ground, the left, and the right. Often the two ground wires are the same color, and the left and right are two more colors.

You can identify the ground either by measuring the resistance between each wire and the end of the jack, or by finding the pair of wires that have no resistance between them. Twist these wires together.

If you care about left and right, check the resistance from the remaining wires to the jack.
The front is left, and the middle is right.
Otherwise, you already have your ground and two sides.

Once you have identified the contacts on the male side, plug it into one of your female jacks and measure the resistance between the known wires and the contacts on the female jack.
It's always a good idea to check in order to make sure there are no shorts and no broken contacts. (so check all of your jacks)

Step 4: Finally...

All that's left to do is solder the two ground wires from the male jack to the ground on the female, left to left, right to right, and finally attach the other female connector(s) the same way.

Remember! If you don't care about left and right, the sides are just sides, because I don't care this meant i could attach my two female jacks pin to pin.