One of the hardest parts of making your own piping is trimming the seam allowance to an exact width.
If you own two rotary cutting mats butt them up against each other. Now spread them apart slightly and push your piping in between the two mats so the cord is sitting in the groove. Place a quilting ruler over the top of the piping and cut to an exact width with a rotary cutter.
In the photo above, to cut 1cm wide allowances I lined up the 1cm mark on my quilting ruler with the stitching on my piping.