I'm reading through and trying to understand the proof of Proposition 5.8 in Chapter II of Algebraic Geometry by Hartshorne that the pushforward of quasicoherent sheaf $\mathcal{F}$ by a morphism $\phi : X \rightarrow Y$ is a quasicoherent sheaf on $Y$ when $X$ is noetherian.
The step I'm actually struggling to understand is the following: we assume that $Y$ is affine, as this is a local quesion on $Y$. Since $X$ is noetherian, it may be covered by finitely many open affines $U_i$, and there are finitely many open affines $U_{ijk}$ covering $U_i \cap U_j$. Then if $V \subset Y$ is open, a section $s \in \phi_* \mathcal{F}(V) = \mathcal{F}( \phi^{-1}(V))$ is determined uniquely by sections $s_i$ in $\mathcal{F}( \phi^{-1}(V)\cap U_i)$ which agree on $\phi^{-1}(V)\cap U_{ijk}$.
Then we get an injection $\phi_* \mathcal{F}(V) \hookrightarrow \bigoplus_i \mathcal{F}|_{U_i}( \phi^{-1}(V))$ by sending $s \mapsto (s|_{\phi^{-1}(V)\cap U_i})_i$ by the identity axiom.
However, we also are supposed to have a morphism $\bigoplus_i \mathcal{F}|_{U_i}( \phi^{-1}(V)) \rightarrow \bigoplus_{i,j,k} \mathcal{F}|_{U_{ijk}}( \phi^{-1}(V))$ with kernel $\phi_* \mathcal{F}(V)$. In a naive way I can see where this comes from, but it doesn't make sense to me as a morphism of rings to me. I think I'm really confusing myself and would appreciate some help clarifying things.