Wrap the ends with some very thin gauge wire to finish the knots. Wedge the wire wrapped ends into the counter sunk holes. Smooth on both sides of the stick. No knot to catch when doing hand stick tricks. If you want to get really fancy, drop a dab of silicone epoxy on the hole to really smooth the stick.
Different tricks work better with different lengths.
You have to plan your series of tricks and find a length that accommodates all your tricks. Nobody can tell you what is correct, you have to discover your own preferences. Slip the string through the hole and tie a multiple knot that is fat enough not to pull back out through the hole. Slip the string through the hole then tie it back onto itself at the side entering the stick. The knot will slide down to tighten itself against the stick.