Establish the parameters of your experiment. For the purposes of this example, consider what would happen if you took three pieces of sandwich bread. Make sure to take them all from the same loaf. Put one on a plate on your window sill (inside the house), so it will receive full sunlight as much as possible. Put a second plate with a piece of bread in your refrigerator and a third plate on a shelf in your garage, or another usually dark space.
Set up a graph with time on the x-axis, in days and mold growths on the y-axis, in number of blotches.
Check each day to see if mold has shown up on the bread. For example, if there are no blotches after Day 1, mark a dot at the intersection (x = 1, y = 0). As days go by, mark the number of new blotches. Take a picture to help you record the data. On Day 2, if there are two blotches, make a dot at (x = 2, y = 2). After Day 3, if there are four blotches, you have two new ones, so mark a dot at (x = 3, y = 2).
Make a column of your y-values at the end of your experiment. Then, total up the numbers in your column. For example, after three days, your sum would be 0 + 2 + 2, or 4.