Making a Punnet square follows the following steps on the basis of two possibilities, one factor cross and two-factor cross:

1. Step 1: Start With the Parents One-factor Cross: Write the genotypes of the two organisms that will serve as parents in cross. In this example we will cross a male and female osprey, or fish hawk, that are heterozygous for large beaks. They each have genotypes of Bb that is Bb and Bb. Two-Factor Cross: In this example we will cross two pea plants that are heterozygous for size (tall and short alleles) and pod color (green and yellow alleles). the genotypes of the two parents are TtGg and TtGg.
2. Step 2: Figure Out the Gametes One-Factor Cross: Determine what alleles would be found in all of the possible gametes that each parent could produce. Two-Factor Cross: Determine what alleles would be found in all of the possible gametes that each parent could produce.
3. Step 3: Line Them Up One-Factor Cross: Draw a table with enough squares for each pair of gametes from each parent. In this case, each parent can make two different types of gametes, B and b. Enter the genotypes of the gametes produced by both parents on the top and left sides of the table. Two-Factor Cross: In this case, each parent can make 4 different types of gametes, so the table needs to be 4 rows by 4 columns, or 16 squares.
4. Step 4: Write Out the New Genotypes One-Factor Cross: Fill in the table by combining the gametes's genotypes. Two-Factor Cross: Fill in the table by combining the gametes's genotypes.
5. 5. Step 5: Figure Out the Results One-Factor Cross: Determine the genotype and phenotype of each offspring. Calculate the percentage of each. In this example, three-fourth of the chicks will have large beaks, but only one-half will be heterozygous for this trair (Bb). Two-Factor Cross: In this example, the color of the squares represents pod color. Alleles written in black indicate short plants, while alleles written in red indicate tall plants.