Glad you asked...
To become blood sisters you will want to make sure your hands are well washed and clean. Then you need to decide what you want to say. Such as a promise to always be good and true friends. You take a sterilized needle, that you cleaned with alcohol and poke your finger and the finger of your friend. Very quickly you hold the two fingers together, with the blood and say your words of promise.
Then again, this answer might help you...
To become blood sisters, each girl takes a sanitary pin, such as a safety pin, and makes a tiny prick on the index finger. Squeeze out a small bead of blood and touch it to each others' fingers where they have pricked their fingers. If you wish to become an official blood sister, be careful to ensure everyone is disease free and your pin is sanitary so you do not contaminate each other.

