The Punnet Square is a diagram used to predict the outcome of a particular cross. In order to so, you must place one of the parents alleles on a vertical distribution and the alleles from the other parentd on a horizontal distribution, as follows:
After that, we cross each allele from one parent with both alleles from the other, filling the blank spaces where those alleles meet, as follows:
Considering the Mendelian pattern of inheritance, BB and Bb are phenotypically dominant and bb is phenotypically recessive. Therefore, 3/4 will have dominant characteristics, meaning 75% (1/4 BB and 2/4 Bb), and 1/4 will have recessive characteristics, meaning 25% (1/4 bb).