I saw these in a department store near me, and thought they looked great, and probably very simple to emulate. And, thankfully, that turned out to be true. After one trial star, I made a few in the hour before my church's Christmas bazaar, and sold them easily.
I started with square patterned craft paper of about 100gsm, but any stiff paper or card would do. If you have A4 or Letter sized paper to hand, then you'll just have larger off-cuts (unless you think ahead slightly and get two smaller stars from a single sheet).
The only tools you'll need are a pencil, ruler and some scissors. And two cut out triangles for each star. Simples.
Here's the pattern that I was using:
To make it draw a vertical line half way across the square (line 1 in the picture). Then with the rule measure the same length as the bottom edge from the bottom right hand corner, until it crosses (1). This is (2). Mark the half way point along (2) at (3), and then measure about 1/15th in each direction, making two (4) marks. Drawn a notch (5) as shown on the opposite edge -- these are what will hold the two halves together. The precise shape and size of the notches aren't critical, but try to make them all about the same.
Repeat until you have an isosceles triangle with three notches. Cut out two such sheets.
Then fold the triangle in half, from each point to the mid-point of its opposite face. If the paper is patterned on one side, like what I was using, then you probably want to turn one of the triangles over before you do the folding. And if you're using thick card, the folding will be easier if you score lightly along the folds first.
The triangles should now look like this:
Then hold the two triangles facing each other, and bring together, mating the notches together.
Then you're almost done. Just make a small hole in the top of one star point, and add a length of brightly coloured thread or string to hang it up. Et voila!
Here are the three I made in about 20 minutes:
On the red and green stars you can see the effect of having one of the triangles turned over before folding. It has meant that the dotty patterns are visible on both parts of the same side of the star, and on the reverse both parts will be blank. You don't have to do it this way, of course ... have fun experimenting with different colour combinations.