Go to the map, find the target dot. Place a mark on the map where it is.
Wait three minutes.
Place another mark on the map where the dot is now.
You can now use the ruler to draw a line from the first dot to the second, and read the target's course off the ruler.
Here, it's 245.
Measure the distance between the marks in yards, divide by 100, and you have the target's speed in knots.
500 yards in three minutes, so speed is 5 knots.