Use tape to secure the first yarn and apply super glue to the rim of your bracelet, to prevent it from sliding down. Wrap the yarn around your bracelet, as close as you can and once you are done tape the end to it as shown in the second image. To change the colour tape the new one slightly above the old one and wrap it around as close, as possible. Once you are done secure the end of the last colour by applying super glue underneath it.
Have fun with your new bracelet.