Slip Knots are especially useful for different types of cast-on methods - they help you get started, by getting the first stitch on the needle.
Step 1: Create a loop with the yarn. Make sure that the tail-end part of the yarn (as opposed to the part of the yarn leading to the ball) is resting on the top part of the junction.
Step 2: Using your right hand, put your fingers though the front part of the loop and grab a bit of the tail-end part of the yarn.
Step 3: Pull a little bit of that yarn through the loop. Don't pull it all through or you will end up with a knot!
Step 4: With your right hand, continue pulling on that little loop of yarn that you had pulled in step 3. As you pull on that piece - and hold the other pieces taut, you'll notice a slip knot forming!! Just stick that loop/knot on your needle, adjust its size by pulling on those 2 other pieces and you're ready to go!
Posted by Jimmy (Laura) of Jimmy Beans Wool