Pearson's Square is a method for determining the exact proportions of feedstuffs needed to produce a specific quantity of nutrient. It involves drawing a square and labeling feeds and their crude protein percentages on the left corners and feeds on the right corners to match. Numbers are then subtracted along the diagonal lines and parts of feeds are divided by the total parts to find the percentage of each feed in the ration. The total pounds needed are then used to determine the pounds of each feed, and the results are checked to ensure the protein needs are met.