Set aside some time when you absolutely will not be disturbed.  A half-hour is good.

This should be done either in silence, or preferably with a fast, steady, regular beat of 4 to 7 beats/second.  Drumming tapes are available, but a little hard to come by.  Best is if you can find someone to drum a steady rhythm for you as you enter the trance.

Lay flat on your back, preferably in darkness.  If the room isn't dark enough, cover your eyes with your arm or with something soft that won't distract you as you journey.  Breathe fully and steadily, let your belly rise and fall with each breath, and get grounded and centered.  If you are working with music or a drummer, now would be the time to start the beat.

You are in a meadow filled with grass and wildflowers.  It is the most beautiful, most peaceful and serene place you have ever seen.  You are perfectly safe here.

Nearby, there is a deep well.  Step over to it, and look in.  Now, take all your cares, all your troubles, all your worries, and visualise them oozing forth from your heart, forming a black ball like a lump of tar.  Drop the ball into the well.  As you hear the "plash" of the ball entering the waters of the well, know that all that negativity cannot follow you where you will soon go....

Look up.  At the edge of the meadow grows a great oak.  This is the largest tree you have ever seen, or even imagined -- the trunk is massive... the branches and leaves stretch beyond the sky above you... the roots reach deep below even the bowels of the Earth.  Go to the oak.  At its base, amidst the tangle of roots, there is an opening, almost like a cave.  It is dark, but the darkness holds no fears for you now, as you have left your own darkness behind at the well.  Enter it.

The opening becomes a tunnel, leading sharply downward.  Follow it.  You may crawl, or walk, or run, or jump, or even fly through the tunnel, as it leads you deeper and deeper, away from the Middleworld you have just left, and into the Underworld.  All things are possible here.  The journey may take some time.  Let it take time.

At last, the tunnel levels, and you emerge into a clearing in a forest.  Small animals look at you curiously, but without fear, for a moment before returning to their business.  Elves and faerie may peep out at you from behind tree trunks or rocks.  You have entered the Underworld, and it is much like the Middleworld with which you are familiar, but everything here is alive and aware.  Magick abounds in this place.

Take a moment to look around you, to see, to hear, to feel, to breathe in the air.  Announce your intent to seek a Power Animal in this place, a companion to guide and protect you, in return for which you will honour this Guide and provide a home for it in the Middleworld.

Now you will want to wander through this magickal forest.  Pay close attention to the animals you see.  Before long, one will appear that seems especially interested in you.  There may be more than one.  Watch it.  Do not follow it, as we do not choose our Guides -- they choose us.  Some of the animals may appear to you more than once.  This means that they are interested in you.  You may not see the whole animal every time -- one time you may see a hoof, or a talon, or an eye, or a horn.

One animal will reveal itself to you three times.  This one is your Guide.  Invite it to come to you, to return with you to the Middleworld.  Grasp the animal gently but firmly with both hands, and hold it to your heart.  It will not struggle.

Now you must return to the Middleworld.  Go back to the forest clearing, to the entrance to the tunnel where you emerged.  Enter the tunnel, and go up... up... up... back through the tunnel until you reach the meadow, at the foot of the great oak, holding your Power Animal to your heart all the while.  Emerge into the meadow.

Now return to your normal state of consciousness, carefully, gently.  If you are using drumming or music, stop them now.  Feel the energy of your new Guardian flow into you through your heart, and spread through your entire body.  It is done.

You will want to honour your new Ally.  Dance the animal.  Move as it would move.  Speak as it would speak.  Don't be self-conscious, merely do whatever you are moved to do.  The dance lets your Ally know that you honour it, and wish it to remain with you.  You will want to do this periodically, to reaffirm your bond with the Spirit Animal that now lives in your heart.

That's it!  :)  You have now retrieved a Power Animal, which will live in your heart so long as you continue to honour it, or until it has a greater need to leave, which sometimes happens even to experienced shamans.  You may retrieve more than one power animal, but never more than one in a single journey, and you don't really need too many of them anyway....

You may find that, in meditation, you are able to speak with your Ally, ask it questions, and receive answers.  It's always good to show affection for your Ally whenever you can feel it, or reach it in any way, to encourage it to remain with you.