Draw a line L on the paper and mark a point A on it.
Take a compass and place its metal point at zero mark of the ruler.
Adjust the compass such that the pencil point gets placed at the point which is 5 small points from the mark of 12cm to 13cm of the ruler.
Now, take the compass to L such that its metal point is on A.
Mark a small mark at B on L corresponding to the pencil point of the compass.
AB is the required line segment of 12cm 5mm.
