To make a slip knot, create a loop in the rope, yarn or other material, secure the loop where the material crosses, make a loop with one free end of the line, and pull it through the original loop. To tighten the knot, twist your right hand clockwise.
A slip knot can also be created using a hook, according to NetKnots.com. Simply place the rope, fishing line or other material through a hook, and make a loop by doubling the line back on itself. Take one of the free ends of the line, create a new loop with it, and wrap the free end two or three times over both lines and through the new loop just after the original loop. Pull the line used to create the knot to tighten the knot. The knot is easy to untie and requires pulling one of the free ends until the knot comes loose.
A slip knot is designed for the knot to slide along the rope. It is commonly used in rock climbing, fishing and knitting. While knitting, the knitting needle is inserted into the loop after the knot is created. AllFreeKnitting.com, Instructables and NetKnots.com contain tutorials with pictures on how to create a slip knot.