Decomposing representations is the process of breaking down a representation of a group into representations of subgroups. In the case of SU(5) decomposing to SU(3) x SU(2) x U(1), the representation with highest weight (0 1 0 0) will be decomposed into irreducible representations of the subgroups.
Here are the general steps to decompose a representation of a group G into irreducible representations of subgroups H:

Determine the weights of the representation of G you are interested in.

Determine the weights of the representations of H.

Find the highest weight of each irreducible representation of H that appears in the decomposition.

Determine the multiplicities of the irreducible representations of H by counting the number of times each highest weight appears in the decomposition.

Write down the decomposition of the representation of G into a direct sum of irreducible representations of H, including the multiplicities.
It is important to note that in general, the decomposition of a representation of a group into irreducible representations of subgroups is not unique, and the decomposition you obtain may depend on the method you use to perform the decomposition.