What you need:
-chain/single strand chain necklace (I used the removable chain from one of my clutches, if you don't have a chain you can easily get one from the craft store)
-pair of pliers
Lay chain flat
Use the pliers' wire cutter to take off about 1/2 centimeter of wire from the hook of one earring.
Hook the earring onto the lowest link on the chain.
Use the pliers to curl the end of the earring hook inward. The hook will now look like a figure-8. The end should be touching the rest of the hook with no gap.
Repeat steps 3&4 with the other earrings, spreading them 2-3 links apart. The spacing is going to be different depending how big the links of your chain are, so experiment a little!
Slightly blurry, but you can see it start to come together
At this point I put the chain on and looked in a mirror as I added more earrings. There's no specific pattern you need to follow--I think the necklace looks best when you just put the earrings on at random. You can even add on stud earrings and pendants to mix it up.
Add on as many or as few earrings as you want, and you're done!
Now just don't lose your necklace!