This is a really good example of how to do single knots: http://www.youtube.com/watch?v=wjjKy5aBsws
Single knotting is great--but when you are starting out with the knotting, sometimes it is difficult to get standard size knots--and further--to get knots that will immediately lay straight like a professional knotted necklace or bracelet.
Now double knotting (or triple, or quadruple knotting) can give you an even knot each time, allowing the stone to have something to lay on, that is almost perfectly square, making a completely uniform knot.
First the supplies: A strand of beads, clamshell terminators (these are just my favorite for the strength as I use some heavy stones--that need a perfect length to lay without weight on your collar bone--hence the 2 inch extender I add to my necklaces), a lobster claw and jumpring (or a clasp of your choice) and bead cord ( I like Griffin Bead Silk so that I can match the cord color with the stone/bead, plus the sizes are great--I stock up on size ten when I am making a stone necklace.).
Now to start the piece--may I suggest trying this with something for yourself first, so that you can learn as you go and not have to redo something you intend on selling?
Step By Step:
Take all the bead cord (if using griffin, just take it all off) you need to knot with--1-2 meters is good for a nice 18 inch necklace and start your first knot by wrapping the cord around your index finger and middle finger twice.
Pull it through, rolling the double wrap over itself so that the top knot is now the bottom and the bottom knot is on the top. After this step you will close the bottom knot (which was the top) first and then close the top knot over it, making a square (shaped) knot.
No put your clam shell on the end of the cord and slide it to the end so that eventually you can close the clam shell over your first knot. Then repeat the knot process on the other side of the clam shell for this look:
Slide a bead or stone on the end of the knot, pushing it down with your fingers or nails to make a tight grip--this is where triple and quadruple knotting may come in. When you have super chunky stones they may have HUGE holes and need more knots so they don't slide around your knotting.
Now knot again on the other side of the bead, add another bead to the cord, and continue:
Once you get the hang of this technique you won't be using your knotting tool or your tweezers anymore. Plus you will notice that your knotting is not only perfectly uniform, but easier to do--making it so you can knot a piece while doing almost anything.
Happy Beading....Here's the Final Product: