DD and I made some for Mother's Day...I posted something similar under the Gift for Kindergarten Teacher post.
We got the very thin black leather string at Michael's in the beading/jewelry making section. I cut the string in about 15 inch lengths. We just picked out beads we liked, in various sizes and put silver "spacer" beads (also found in the jewelry making section) in between the beads. Tied a knot at one end of the string, put about 1.5 inches of beads on in a pattern we liked, tied another knot to keep the design from sliding and basically did the other end the same way so we wound up with about a 12 inch cord with about 1.5 inches of beads at either end (the beaded part to hang out of the book and the string part to lay inside the book along the spine) Hope that makes sense. We didn't have a pattern or instructions, I had seen one a mom at school made for the teachers.
My mom also has one a friend in Bible study made her but it was beaded all the way down the string and mom said the book wouldn't close flat because of the beads being all the way down and she liked the way we did it better with just the string part in the middle.
I think we paid like $2.99 for 3 yards or so of the string and got a big set of beads for about $5.99 they turned out very cute and very economical.