Formal Grammar: derivation form posted on Wiki?
You seem to be looking for a deterministic procedure that isn't there.
With multiple production rules, you solve in every possible way.  For instance, for the grammar
$S \rightarrow a \\ S \rightarrow aS$
we have
$
\underset{G}{S \implies a}, \\
\underset{G}{S \implies aS}, \\
\underset{G}{aS \implies aa}, \\
\underset{G}{aS \implies aaS}, \\
\ldots
$