All methods of casting on begin by the creation of a slipknot. Draw a length of yarn from the ball/skein, approximately 1" for each stitch you wish to create, though this varies with Needle size. Now, lay the yarn out such that the loose end is to your left, and then form a loop by twisting the yarn once, clockwise:

Slip Knot Step 1

Next, turn the loop clockwise once more and lay it over the loose section of yarn as follows:

Slip Knot Step 2-A Slip Knit Step 2-B

Now, place the needle through the loop as shown:

Slip Knot Step 3

And tighten the loop on the needle to form the knot:

Slip Knot Step 4