Tying a slip knot involves creating a loop with the string and inserting another loop. The key characteristic is that pulling from the tail tightens the loop, while pulling from the top of the loop loosens it. An instrument, like a crochet hook or dowel, for example, is necessary to ensure the knot doesn't come untied.
Begin by pulling a 12-inch length of string, and grasp that point between your thumb and index finger of either hand, with the palm facing you. The tail of the string should be on the palm side of your hand. Drape the other end over the back side of your fingers. Grasp near the tail of the string, and pull it over the front of the string behind your fingers.
Holding the tail end with the last three fingers of your free hand, wrap the tail end around the back side of the connected end, and use your index finger to create a loop. Push the loop with your index finger through the knot created around your other hand, along the back side of your fingers, and remove your hand from the knot.
Grasp both ends of the string with one hand, grab the loop with your other hand and tighten. Pull on the tail of the string to shrink the loop on your slip knot.