Gather supplies:

15-20" waxed thread
assorted beads, spacers, & charms
ruler (or book to measure by), scissors, split ring
(needle-nose or round-nose pliers also come in handy)
clear nail polish

Measure string to desired length. Add 5-7" for beads & knots.

Add charms and/or beads to your split ring.

Tie one end of the string to the split ring. I recommend two knots, cut the excess then seal with nail polish.

Add beads!
Tie another knot at the top of these beads. You do not need to double-knot or seal this knot, but feel free to do so if you choose.
Here I use the pliers to help keep the beads in place so I can tie the knot real close.

Measure 8-11" (or length of book) from the knot, tie another knot. (Paperbacks are about 7"-8.5", Textbooks 10.5"-11", my largest book is 14")

Add more beads!
This will be the bottom of the bookmark, so I recommend fewer and/or smaller beads here.
Tie another knot real close & cut off excess string.
(Similar to the first knot, I recommend double-knotting and sealing with nail polish.)

Et voila! You're done!