Since the single-strand simple knot is so simple, this is more about usage than how to tie it! The main use I find for it is to prevent fraying near a free end of kite line, in just about any situation. Of course, for nylon or polyester lines there is also the old kite-maker's trick of melting the frayed end with a flame. The gas stove lighter can come in handy for this.
Another handy use is to fine-tune the amount of bow in a horizontal spar. Wait, I'll explain! The MBK kite designs use fixed-length toggled bow-lines to put some curvature in horizontal spars. To increase the amount of curvature, you can put knots in the lines. One at a time, until the amount of bow is where you like it. Actually, it's surprising how much difference just one knot makes!
I tried using this knot to prevent Slip knots from pulling
through, but it's not really big enough. More recently, I have been using the Double knot instead - which is much bigger but far neater-looking than the small Loop knots I used to use! As long as you don't leave too much line hanging free from the knot.
Oh yes, one more situation... The shoe-lace ties used by the kites in the Dowel Series fray really badly where cut with scissors. A tight knot near the cut end of the shoe-lace will stop the fray from getting worse. Quick, easy and effective. The specified length to make the ties takes this shortening of the tie into account.Now, if you really want to go overboard with knot-tying...
The main use for the Multi Strand Simple knot in an MBK kite is tying off the lines that have been taped to either side of a plastic keel. The nice thing about a double-line knot is that you can adjust it to an exact position along the line, before you tighten it fully. It takes some practice, but it feels good when mastered!
With a little care, this is a great knot to use along the edge of the keel that touches the vertical spar of a kite. If the knot is adjusted too far one way, you'll crush the keel edge out of shape. Too far the other way, and there will be an untidy gap between the keel edge and the vertical spar when you finally attach the keel to the spar with Granny knots.
This knot is also handy at the Towing Point corner of a keel, defining the spot through which the flying line tension acts.