 Solution

Form the presheaf whose value on $U\subseteq X$ iswith restrictions induced from those of the two sheaves of modules and the structure sheaf. Its sheafification is the tensor product of sheaves $\mathcal{G}{\otimes }_{{\mathcal{O}}_X}\mathcal{H}$. Sheafification is needed because taking a tensor product on each open set need not itself satisfy the gluing axiom. The resulting sheaf represents bilinear sheaf morphisms, and localization gives the stalk formulaIndeed, sheafification preserves stalks, and filtered direct limits commute with the module tensor product.