First decide on the size and shape you want the spikes to be and cut out a pattern (cardstock works well).
Next figure out how many spikes you will have. This depends on the size and spacing. Mine are about 3 inches wide at the base and will be about 1 inch apart, so I estimated that I needed 10 spikes. (*Remember that they will be a good deal smaller after sewing. 10 ended up not being enough, so I had to make extras.)
Now trace your pattern onto the fabric and cut out twice the number of spikes (*2 sides to each spike).
Pair up your triangles and lay them face to face (wrong-side-out), then sew the top two sides- but not the base. Pin together if necessary.
When all this is done, turn the little triangle pockets right-side-out and you're ready to put it all together.