All right! First off, you are going to want to find the seam where your putting the zipper (usually down the middle of the back). If you tug at it a bit, you can usually slip the end of your scissors and cut the string, and un-do the back from there. Or, if you choose to cut your own hole where there is not a seam, then be carefull that you make it straight for the zipper later on.

Next, pull out as much or as little stuffing as you desire. I pull out most of the torso stuffing, but still keep a small amount to line the torso to help keep it's shape.
Next, measure the pocket and opening and make a pocket to line your animal so your belongings arn't lost within your animals stuffed innards. you can either make a square, or a trapizoid like below, depending on how much room you want for your things. sew the 2 sides together, and the bottom.

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NOTE: in the following pictures, i messed up and i pinned the fabric first and then the zipper on top. either way is all right but if you pin the zipper first, then the fabric pocket, the zippers ends will not be shown and it will look a little bit more put together. I just wansn't thinking and i messed up. lol
How it should have looked:

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Next, you will want to pin on the zipper.
First you will want to unzip the zipper and pin it to the fabric facing outwart. (when you zip it up, the messy ends of the fabric on your animal will not be shown)

then, pin pocket on top of that, and sew on. Repeat to other side.

When you sew it on, your going to keep it unzipped unless you are sewing by hand or have a zipper foot for your sewing machine. if you are using a sewing machine with no zipper foot, then you will notice that there will be a spot near the bottom you cant sew. NO BIGGIE! it takes a minuet to finish by hand when your done.