Step 1: You need
- some patience
- some sewing ability
- a long zipper
Buying a long zipper might be rather expensive. Or you can be ingenious: I got mine by unsewing it from a plastic clothe cover I bought for 3$.
Step 2: Create the bottom of the bag
Then, put the plug end of the cord between the two layers and sew along the other edge.
Step 3: Next turn
Step 4: Keep sewing past the divergence
Notice that, because of the creation of the bottom, the strips have now different lengths. So, if one of your earbuds cords is longer than the other, sew it inside the longer strip.