  We give a constructive proof of the theorem of Bass and Milnor saying that if
$G$ is a finite abelian group then the Bass units of the integral group ring
$\Z G$ generate a subgroup of finite index in its units group $\U(\Z G)$. Our
proof provides algorithms to represent some units that contribute to only one
simple component of $\Q G$ and generate a subgroup of finite index in $\U(\Z
G)$ as product of Bass units. We also obtain a basis $B$ formed by Bass units
of a free abelian subgroup of finite index in $\U(\Z G)$ and give, for an
arbitrary Bass unit $b$, an algorithm to express $b^{\varphi(|G|)}$ as a
product of a trivial unit and powers of at most two units in this basis $B$.
