There are many,many ways to piece that border. Some have been mentioned above.
I usually piece them with setting triangles - you can see the units outlined in blue below. You have the diamond shape (as one piece of fabric), and then setting triangles just as you would if you were dealing with squares (although the shape is different). But you do have to deal with some bias.
Another option would be to paper piece it. This would guarantee that your angles are perfect, and the diamonds meet tip to tip.
A foundation like the one below could be used. Just cut the corners over-sized. Then you can trim the unit to a perfect rectangle of 2.5" x 4.5".
If you leave the paper on while you're putting together the border, it should keep it stable and straight. Also, you wouldn't have any fabric in the seam allowance (always a plus) to struggle taking off.
And you wouldn't need a special ruler.
Like I said, there are many, many different methods.