Like many, I started making crown sennits as a kid with those flat plastic spaghetti things, a clean finish was not simple until I learned the wall knot, which is simply a crown going backward. Variations and combinations of the crown and the wall form the basis of many decorative knots, the diamond, the star, the Mathew Walker, the little lump...
A google search for "knot crown wall" (without quotes) should get sites with instructions and illustrations, for example:
http://www.home.zonnet.nl/willeke_igkt/tables/fob_table.html
http://www.scoutxing.com/knots/crown/crown.htm
http://www.scoutxing.com/knots/wall_knot/wall_knot.htm
A good place to look around is the site of the International Guild of Knot Tyers:
http://www.igkt.net/index.html
It's complicated to explain with words, but for anyone interested let me try to describe a way of making a simple fob:
Get two pieces of cord about 3 feet long each, fold them over so that you get four strands, leave a loop about two inches long where they fold and hold the four strands together with a wire tie, rubber band or whatever.
Begin the fob with a single diamond knot, here's a way to tie it (the illustrations are with three strands, four is similar):
http://www.diamondknot.com/brewery/knot.htm
Or do it in two parts, first a crown then a wall, etc., as I'll try to explain:
Tie a crown and leave it a bit loose, then tie a wall, notice how the wall tends to go under the crown made previously, look at it from the top, see that there are four places where two strands cross each other forming the crown, take each loose strand and stick it from underneath between the two closest crossing strands and pull it out and up through the center of the crown, once all four strands are up tighten the whole thing by pulling up on each strand, go one by one pulling up several times until you feel it's tight.
Now remove the wire tie or rubber band and see how it holds together.
Now do a series of crowns, one after the other tightening each one as you go, forming the crown sennit, stop while you still have at least 6 inches free on each strand.
To finish make another single diamond knot, the same as when you started, make it tight. To make sure that the end holds pull the strands apart and add a drop of super glue right in the center and as deep inside as you can.
Now pull up hard on one of the strands and cut it as close to the top of the knot as possible with a sharp knife or razor blade (careful, don't cut yourself), pull on the other strands and push the top of the knot together to hide the cutoff end inside, do the same with another end, pulling the leftover strands and pushing the end knot close to hide the cutoff end, repeat with the third strand and finally pull and cut off the last strand and push the top of the knot closed.
Finished, I hope it is more or less clear.
Luis
Edited to change the picture. These fobs are in macrame cord. On the left, like mentioned but I did not hide the end strands, instead I unraveled them and trimmed them as a tassel, this way it does not need super glue to hold. On the right, two colors, a single loop holds a small spring shackle, the second loop is hidden inside the knot, the crowns are done alternating directions, I made a Mathew Walker knot in the middle for decoration and the end knot is a double diamond.
