Let $X = V / \Lambda$ and $X' = V' / \Lambda'$ be two complex tori, of dimensions $n$ and $n'$. If $f : X \to X'$ be a homomorphism, then $f$ is induced by a linear map $F : V \to V'$ which satisfies $F(\Lambda) \subset \Lambda'$. I want to know how we find $F$ given a morphism between the lattices $\Lambda$ and $\Lambda'$.
More explicitly, I'm trying to see the map $F$ induced by an isomorphism of the lattices $\Lambda$ and $\Lambda'$. For example, if $X$ and $X'$ are both one-dimensional (i.e. marked elliptic curves), then $X$ and $X'$ are isomorphic if and only if there is a $\gamma$ in $SL_2(\mathbb Z)$ such that $\gamma \Lambda = \Lambda'$.
Write $\gamma = \begin{pmatrix}a & b \\ c & d \end{pmatrix}$. We can find $\tau$ and $\tau'$ such that $\Lambda = \mathbb Z \oplus \tau \mathbb Z$ and $\Lambda' = \mathbb Z \oplus \tau' \mathbb Z$, and in this case $\tau' = \frac{a \tau + b}{c \tau + d}$. I have some lecture notes ("Lectures on moduli spaces of elliptic curves", R. Hain) which say the linear map which induces the isomorphism $X \to X'$ in this case is $L : z \mapsto \frac 1{c\tau + d} z$, without going into detail on how this is calculated.
Question: How do we find the map $L$ given $\gamma$?