Place the acetate with the dot grid, over the area that you want to measure. It doesn't matter how the acetate fits over the image, as long as all of the area to be measured is covered by the dots.
Count the dots that fall within the area to be measured. Once you start counting, don't move the acetate. If a dot is on the edge of the area, count it if it's more than one half inside the area, and don't count it if it's less than one half inside the area.
The number of dots that represent 1 square kilometre will be specified in each activity. Divide the total dot count that you made by that number to arrive at the final area in square kilometres.