A Punnett Square shows the genotypes that two individuals can produce when crossed. 

So, to draw a square, write all possible allele combinations where one parent can contribute to its gametes across the top of a box. 

Then, write all possible allele combinations from the other parent down on the left side. 

The allele combinations produced along the top and all the sides become labels for rows and columns within the given square. 

Now, complete the genotypes in the square by filling it in with the alleles from each of the parents. 

The idea here is that since all sorts of allele combinations are equally likely to occur, a Punnett Square predicts the probability of a cross producing each genotype.

A single trait of Punnett Square tracks two alleles for each parent. The square has two rows and two columns that makes it a perfect square. By adding more traits, you can increase the size of the Punnett Square. 

Assuming that all traits exhibit some independent assortment, the number of allele combinations an individual can produce is 2 raised to the power of the number of traits. Multi-trait Punnett Squares are large. A three-trait square has 64 boxes. A four-trait square has 256 boxes and so on.