The shape of membrane and cable-net structures is usually modeled by geometry of minimal surfaces. Using central finite differences method, a nonlinear iterative process for finding the minimal surface with given fixed boundary conditions is developed and implemented in $\text{Mathematica}^\circledR$. Having in mind form-finding of membrane structures, the results are compared with the results obtained by commercial package $\text{EASY}^\circledR$, made by Technet GmbH, Germany.