Begin with a square or rectangle of your border background fabric (foundation fabric)...you choose the size, but remember one side (measurement) will need to be consistent, so that when you sew the border pieces together you have a consistent border width....with little trimming after. For example you could cut the foundation pieces 2.5inches X 4.5 inches, 3.5 X 4.5 inches, etc.
Take a piece of scrap fabric for the spike...it just needs to be large enough that you can sew it on to the corner of the foundation and fold it back to completely cover the corner of the foundation piece.
Place the scrap diagonally across the foundation fabric (right sides facing). I usually try to cover the base of the foundation piece with the scrap and the length or spikeyness (new word) of the spike depends on how far up the adjacent side you take the scrap fabric. Some of my spikes are tall and thin and some are short and fat!
As you can see the spike will lay on the background fabric from the base to the right side about 2/3rds of the way up the side.
Now sew a 1/4 inch seam using the scrap as your raw edge.
Fold the scrap back over so the right side is facing you. Finger press and then press with an iron. Flip the whole block over so you are looking at the back of the foundation and the scrap piece. Use the foundation fabric as a guide to cut away the excess scrap fabric. You can trim the foundation fabric away at the 1/4 inch seam.
Continue making spikey blocks until you have enough to piece together and cover the sides of your quilt top.
In this picture I have made the long pieced border a little heavy at the base. I would probably just trim the base before adding to the quilt.
Hopefully, this makes sense....just leave a comment if you need clarification! I love to hear from you!
**shout out Chris**