Step 1: This Is What You Need:
Needle and thread or sewing machine (I'm too impatient to use a needle and thread :P) You'd probably be better off using grey thread.
Fiber fill or some random bits of material and yarn since you used up all your fiber fill making an xkcd pillow. This is optional, however.
A ninja star printed off of the Internet. I went with a four pointed one for this, since it was very simple.
Step 2: Chop Chop
For this I'm not even going to deal with having to turn it inside out and stuff, since that would just be kind of a pain in the butt. So it really doesn't matter about the right sides out or anything like that.
So cut out two ninja star shapes.
You may want to draw the circle you're supposed to cut out onto the material, since it's kinda hard to cut out a good circle when you're trying not to cut up the paper.
Step 3: Well Well Well...
Once you finish sewing the inside together, you must sew the outside together too. Unless of course you don't want to. But whatever, anyway sew along the outside (and try to sew as close to the edges as you can) until you get to where you want the loop for hanging on the tree to be. Make a loop with the ribbon and put the two ends of it in the star and just sew over them.
Don't worry if the sewing is a bit... off if you're using a sewing machine, it'll definitely take time to get used to making these. Try to sew in straight lines though! If you go off the material a little bit don't try to fix it until you've sewn the rest of it or it might turn out weird.
For the part you will put the stuffing in, I suggest you use a side instead of a point, since that will make it much easier to put the stuff in and keep it in while you're sewing it completely shut. You really won't need much to stuff it with, especially if you save your scraps of fleece you cut out. If you wanted to you wouldn't even really have to put anything in it at all. In fact, I think they look better without anything in them.
After you stuff it (or don't stuff it) sew up where you put the stuffing in. Then you could cut off the excess pieces of material around the edges if you don't like the look of it.
Step 4: Hang It on the Tree!
If you want to give these as gifts you could embroider someone's name on it, and the year or whatever you want. But you might want to do that before you sew them up.