Step-1 Many objects could be used inside the knot preferably round or square but I chose to use a marble. The marble is light weight and you can get it in different sizes.
Step-2 I recommand using some sort of pointy object I chose my nail punch. Once the knot is finished the pointy object will aid you when pulling the cord to tighten the knot around the marble.
Step-6 Insert the marble in the center of the knot.
Step-12 I made a diamond knot and snake stitch but any type of finishing could be applied.
The first few attempts could go wrong - this knot needs a LOT of patience so take your time and especially the final tightening part, do it slowly and you will get a cool Monkey's fist.
In the second guide I am just showing another method of holding the cord between the fingers. It is convenient way for small marbles.