For a simply connected connected simple algebraic group $G$, a cell $B_{w_0}^-=B^-\cap U\overline{w_0}U$ is a geometric crystal with a positive structure $θ_{\textbf{i}}^-:(\mathbb{C}^{\times})^{l(w_0)}\rightarrow B_{w_0}^-$. Applying the tropicalization functor to a rational function $Φ^h_{BK}=\sum_{i\in I}Δ_{w_0Λ_i,s_iΛ_i}$ called the half decoration on $B_{w_0}^-$, one can realize the crystal $B(\infty)$ in $\mathbb{Z}^{l(w_0)}$. By computing $Φ^h_{BK}$, we get an explicit form of $B(\infty)$ in $\mathbb{Z}^{l(w_0)}$. In this paper, we give an algorithm to compute $Δ_{w_0Λ_i,s_iΛ_i}\circ θ_{\textbf{i}}^-$ explicitly for $i\in I$ such that $V(Λ_i)$ is a minuscule representation of $\mathfrak{g}={\rm Lie}(G)$. In particular, the algorithm works for all $i\in I$ if $\mathfrak{g}$ is of type ${\rm A}_n$. The algorithm computes a directed graph $DG$, called a decoration graph, whose vertices are labelled by all monomials in $Δ_{w_0Λ_i,s_iΛ_i}\circ θ_{\textbf{i}}^-(t_1,\cdots,t_{l(w_0)})$. The decoration graph has some properties similar to crystal graphs of minuscule representations. We also verify that the algorithm works in some other cases, for example, the case $\mathfrak{g}$ is of type ${\rm G}_2$ though $V(Λ_i)$ is non-minuscule.