Thread your yarn needle with the main color you choose. You may cut your yarn about 4-5 feet long. Usually I just pull a bunch of yarn through the bead, but leave it connected to the yarn skein until I am nearly done wrapping my bead. Do what you feel is best. If you cut it too short, just add another section of yarn, hiding the end inside the hole. No one will know, so don't worry about how much yarn you begin with. The amount of yarn you actually need will depend upon the thickness of your yarn, as well as the size of your bead.
Feed your yarn-threaded needle through the hole of the bead.