So here's how you can keep your threads untangled -- only using supplies that I would be very surprised if you don't have around your house.
Step 1: Materials
1. Some posterboard-weight cardboard.
I used the wrapping from a blanket I bought. You want it to be sturdy; an index card is too light (believe me, I've tried -- they just end up buckling!) and you can go as heavy as you want, so long as the thread can wrap around it well without being caught on anything.
3. The thread you want to wrap
I'm wrapping some DMC embroidery floss in this tutorial, but I've also used this method for wrapping regular crochet thread.
4. Optional: Hole punch
I like the option of being able to put my bobbins on a ring. But if you don't care, you can skip this one.
5. Optional: Pen or pencil
I like labeling the bobbins with the color code so I can replace it with the same one when I finish that bobbin.