Phenotypes and Genotypes?

I'm confused. So for our homework, we had to list all the possible genotypes for blood groups ABO, and we have to indicate the phenotype for each. How do you do that? I started off with the square graph thing, and I wasn't sure what to do after that. I called my friend, and she said that I was supposed to make ratios for phenotype and genotype. I didn't know how though. She said that the genotype should be like 1:2:1, and the phenotype should be like 3:1 or something. How do you do this?