The Monkey Fist is one of the most popular knots made with paracord. It works great as a weight at the end of a line in addition to serving decorative purposes on fobs and such. Monkey Fists can be made in sizes ranging from small to large depending on the purpose you have in mind for yours.
Monkey Fists are also difficult to tie until you get the hang of it. The basic knot structure is not difficult so much as holding and pulling all of the right cords at the right time. Some people use jigs and such to tie Monkey Fist knots, but I never have. Perhaps I should, but impatience has kept me from doing so; I would much rather try to do it my way right away than look for an easier method. Thus, I have always had a bit of a love-hate relationship with the monkey fist. I would love to be able to tie some of the beautiful ones I see others tie but my hands hate for me to try. I cannot grasp the cords very well with my weak hands. While you may not have the same problem and tying a monkey fist may come second nature to you, I can only tell you what it is like from my perspective...and it is not always easy.
To tie a monkey fist, you need to first determine the size you want it to be. The bigger the monkey fist, the more strands you will need. For the sake of learning, however, it is easier to start small because if you mess up, there is less cord to untie so you can try again. With that in mind, I recommend starting with only two loops.
Around your fingers, make two complete loops of cord with the starting end of your cord hanging down beside them. Hold your first two loops vertically and make two horizontal loops around them. Then take your cord and make two more loops that will go around the horizontal cord and through the vertical cord.
Being tightening the loops, working them closed to cinch down your monkey fist. The easiest way I have found to do this is to start with the original end-tuck it inside the monkey fist and begin your tightening with the slack of that piece of cord. Make sure the end stays tucked inside (sometimes tying a small knot it in the end will help with this) as you go. Work the loops tight in the same order that you created them and when you are finished, you will have a complete monkey fist.
The monkey fist can be quite frustrating the first few times you attempt to create it and there is sure to be some trial and error. As you practice, however, you will get the hang of it and become able to create a well-defined monkey fist. The more your skill develops, the larger the monkey fist you will be able to tackle, just remember to increase the number of loops by the same number at every angle (horizontal, vertical, etc.). Larger monkey fists sometimes require items to be placed inside to hold their form, however, so before you attempt monkey fists on a large scale, invest in a few round objects for experimentation purposes. Marbles or golf balls are lightweight and easy to maneuver while still enabling you to get the hang of where to position them inside the fist.
Many people tie monkey fists with steel balls or other such objects inside but check local laws before carrying such an object as this sort of thing can be illegal. If you do make a monkey fist intended for use as a weapon, keep it out of the reach of children as well as anyone who might turn around and attempt to use it against you. A well-placed whack to the head with a monkey fist containing a heavy object can do serious harm or possibly even kill someone, so if you carry and someday use one, always keep in mind the damage that can be done with what you wield as well as your responsibility for that damage after the fact.