I have noticed that there are quite a few people who have needed this pictorial tutorial. This stitch is essential when making stuffies, and is even used to repair clothes that have torn, especially if they are lined and you don't want to take them apart to repair them.
This may be what your opening may look like:
You'll need to close the opening. I think about closing lips. I finger press the edges so that they have a better crease:
Next, you'll pin the edges closed, or use hem tape to close the lips temporarily, while you sew. If using the pins method, make sure your pins are perpendicular to the opening. This means, your opening will be horizontal, and you pins will be vertical.
Now you can begin sewing. Insert you needle from underneath. You can do this by opening up the gap at one end and inserting your needle from inside.
Crossing straight over to the other side, (this is important that the stitch be straight across. If not, you'll be able to see your stitches), take a very small stitch on the bottom row. This stitch will be approx. 1/4 centimeter. I'm not kidding here. It needs to be small! If it is not small, you'll end up with puckering.
Pull taut, not tight. If you pull tight, you'll be gathering the fabric. Cross straight over to the other side and take a small stitch. I this picture I haven't pulled the thread taught yet, so that you can see the previous stitch.
You now have two stitches. Cross over and take another stitch. Repeat this all the way across.
When you get to the end, tie off your thread and then poke your thread into the project, exiting anywhere far from the knot. Pull thread tight and cut close to the fabric. When you let go, the thread will be lost inside the project, and you won't have to worry about your knot coming untied.
I hope this helps!
Edit to add: pics of how to pin a stuffy for this stitch.