The first step to making a punnett square is to find the parent traits and what there notation is. Also, you have to know whether the trait you are dealing with is Autosomal or X-Linked. Once all of this is done put the parents on two adjacent sides of a square and begin to cross the parents:
http://passel.unl.edu/Image/siteImages/PunnettSquarePlainLG.gif
After completing the cross you can begin you analyze the genotypic ratios as seen by the punnett square. As you can see AA is representing 25% of the possible trait. Aa is representing 50% of the possible trait. Lastly, aa is representing the last 25% of the given trait. It may also be asked to be written as ratio so it would be AA:Aa:aa or 1:2:1. Next, when you have a given trait and it asked for possible phenotypic ratios you will want to combine the Homozygous Dominant with the Heterozygous. This is because they are both dominant and it will have the same outcome as a trait. Different situations will bring different things to find. Although, since you now know the basics and more, click the "Practice Punnett Squares" tab to get used to solving them and try them on your own.
After completing the cross you can begin you analyze the genotypic ratios as seen by the punnett square. As you can see AA is representing 25% of the possible trait. Aa is representing 50% of the possible trait. Lastly, aa is representing the last 25% of the given trait. It may also be asked to be written as ratio so it would be AA:Aa:aa or 1:2:1. Next, when you have a given trait and it asked for possible phenotypic ratios you will want to combine the Homozygous Dominant with the Heterozygous. This is because they are both dominant and it will have the same outcome as a trait. Different situations will bring different things to find. Although, since you now know the basics and more, click the "Practice Punnett Squares" tab to get used to solving them and try them on your own.