Slip Knot

This is the best line drawing of a a slip knot I've seen:

A: Loop the yarn around and let the loose end fall behind the loop to form a pretzel shape as shown. Insert the hook.

B: Pull both ends to close the knot.

However some people do better understanding photos than drawings.

Here is how to make a slip knot in "slow motion" photos. First lay the ball of yarn on a table and draw some of it out so you can manipulate it:

Curl the end of the yarn into the shape of a backwards letter "C."

Hold the yarn at the X, pick it up, and bring it down in the direction of the arrow.

You now have a loop like the photo below:

Hold the yarn at the X, pick up the whole loop and lay it over the free end of the yarn.

You will get a pretzel shape like this:

Hold the yarn at the X and carefully start to pull it through the loop while pulling the ends in the opposite direction.

You will get a shape like this as you pull:

As you keep pulling it will tighten more...

...and more:

Stick a crochet hook in the loop. If done correctly, tugging one side will cause the loop to shrink. This is how you can tighten it around a crochet hook.

Now that you've make the slip knot, you are ready to chain.