Determine the frequency of each number in your data list. Frequency means the number of times a number appears in your data set. For example, consider a data list with the numbers 3, 3, 5, 5, 5 and 6. The number 3 appears twice, 5 appears three times and 6 appears once. Thus, the frequency of 3 is two, the frequency of 5 is three and the frequency of 6 is one.
Draw a horizontal and vertical axis for your graph. The horizontal axis contains each number in your data set, while the vertical axis displays the frequency of each value. In the given example, the horizontal axis goes from "1" to "6" and the vertical axis goes from "1" to "3."
Make a bar on the graph for each data value and its frequency. Since the number 5 appears three times, a bar would start at "5" on the horizontal axis, then rise up to "3" on the vertical axis. Each bar should have the same relative thickness to every other bar on the graph.
Find the midpoint on the horizontal axis and fold the graph vertically along the midpoint. If the shape of the graph before the midpoint is the same as the shape of the graph after the midpoint, the graph -- and thus, the histogram -- is symmetrical.