A cutting mat with mm markings for accurate cutting.
A metal ruler. It's better if heavy. If your ruler is curved, straighten it and have the curve facing down on the mat for a better grip.
Sharp box cutter. Make sure only the tip juts out and that it is sharp. If blunted sand paper it sharp. Sand the flat side holding sand paper flat, and for the angled side of the blade hold the sandpaper with the angle to get a razor sharp edge.
Next, align your paper using the grid lines on the cutting mat.
For accuracy you may mark the required interval at the top and bottom of the paper and use the markings and the grid of your cutting mat to align the ruler.