1. The Punnett square should be laid down in a table of two entries. On the top entry, the possible alleles of one of the parents, and on the other entry, the possible alleles of the other parent, which will be the same as both have the same genotype. The possible alleles for both parents would be: DC, Dc, dC, and dc. On the actual table's body, all alleles should be crossed from one entry and the other obtaining the possible genotypes of the offspring and their proportions, and, therefore, the possible phenotypes and their proportions.
2. By the Punnett square (attached is an example of it), we are able to identify that all possible genotypes have a 1/8 likelihood to happen, besides the genotypes that are homozygous for both alleles (whether dominant or recessive on both traits, or dominant on one trait and recessive on the other) which have a 1/16 likelihood to happen, and those that are equal to the parents (dihybrid) with a 1/4 likelihood to happen.
DDCC, 1/16; DDCc, 1/8; DdCC, 1/8; DDcc, 1/16; DdCc, 1/4; Ddcc, 1/8; ddCC, 1/16; ddCc, 1/8; ddcc, 1/16.
3. Dark and curly hair corresponds to 9/16 of all the offspring. These traits are dominant, and, therefore, are the ones that are present depending solely on the presence of one dominant allele on each trait.
Dark and smooth hair have a likelihood of 3/16 - all those that are homozygous recessive for smooth hair but still have, at least, one dominant allele for dark hair.
Light and curly hair have a likelihood of 3/16 - all those that are homozygous recessive for light hair but still have, at least, one dominant allele for curly hair.
Light and smooth hair have a 1/16 likelihood to occur, which corresponds to the only possible full homozygous recessive genotype.