[step:Characterize the dual Dolbeault operator by graded evaluation]
Let $U\subset X$ be an open set with holomorphic frame $e_1,\dots,e_m$ for $E|_U$, and let $e^1,\dots,e^m$ be the dual holomorphic frame for $E^*|_U$, characterized by $e^i(e_j)=\delta_{ij}$. For $\lambda\in A^{p,q}(U,E^*)$, write
\begin{align*}
\lambda=\sum_{i=1}^m \lambda_i\otimes e^i
\end{align*}
with $\lambda_i\in A^{p,q}(U)$, and define locally
\begin{align*}
\bar{\partial}_{E^*}\lambda=\sum_{i=1}^m \bar{\partial}\lambda_i\otimes e^i.
\end{align*}
This definition is independent of the frame. Indeed, if $e'^1,\dots,e'^m$ is the dual frame corresponding to another holomorphic frame $e'_1,\dots,e'_m$, then the transition matrix for $e'^1,\dots,e'^m$ is obtained from the holomorphic inverse transpose of the transition matrix for $e_1,\dots,e_m$. Its entries are holomorphic, so applying scalar $\bar{\partial}$ to the transformed coefficients produces no transition-derivative terms.
Let $s\in C^\infty(U,E)$ and write
\begin{align*}
s=\sum_{i=1}^m s_i e_i
\end{align*}
with smooth functions $s_i:U\to\mathbb{C}$. The graded evaluation convention is
\begin{align*}
\lambda(s)=\sum_{i=1}^m \lambda_i s_i
\end{align*}
and
\begin{align*}
\lambda(\bar{\partial}_E s)=\sum_{i=1}^m \lambda_i\wedge\bar{\partial}s_i.
\end{align*}
Using the scalar Leibniz rule and the fact that $\lambda_i$ has total degree $p+q$,
\begin{align*}
\bar{\partial}\bigl(\lambda(s)\bigr)=\sum_{i=1}^m \bar{\partial}(\lambda_i s_i)=\sum_{i=1}^m(\bar{\partial}\lambda_i)s_i+(-1)^{p+q}\sum_{i=1}^m\lambda_i\wedge\bar{\partial}s_i.
\end{align*}
Thus
\begin{align*}
\bar{\partial}\bigl(\lambda(s)\bigr)=\bigl(\bar{\partial}_{E^*}\lambda\bigr)(s)+(-1)^{p+q}\lambda(\bar{\partial}_E s).
\end{align*}
Conversely, if an operator $D:A^{p,q}(X,E^*)\to A^{p,q+1}(X,E^*)$ satisfies this identity for every smooth section $s$, then evaluating against the local frame sections $e_j$ gives
\begin{align*}
(D\lambda)(e_j)=\bar{\partial}(\lambda(e_j))=\bar{\partial}\lambda_j.
\end{align*}
Hence $D\lambda=\sum_j\bar{\partial}\lambda_j\otimes e^j=\bar{\partial}_{E^*}\lambda$, proving uniqueness.
[/step]