I don't understand how to do a dyhybrid cross?

zygous genes have diploid gene pairs, meaning that each has identical alleles in both copies. In your question this means that the black pacer has a recessive diploid gene pair tt, while the chestnut trotter has the dominant diploid pairing, TT. Doesn't matter where you put them in the Punnet square, but the dominant pair usually goes on the top, while the recessive pair usually goes on the left. This means the offspring will all have Tt heterozygous genotypes, which will result in trotting phenotypes (the physical expression of the genotype).