Here's an example of using the relaxation method with EXCEL.  The first table is the intial state. Two of the walls are grounded and two have potential = 1.
Before
 1 1 1 1 1 1 1 1 1 1 1 0 1 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0 1 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0 1 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0 1 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0 1 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0 1 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0 1 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0 1 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0 1 0 0 0 0 0 0 0 0 0 0 0
Now we perform the averaging to obtain the potential in the interior of the region.
After

 1 1 1 1 1 1 1 1 1 1 1 0 1 0.975943 0.952772 0.930827 0.909723 0.888161 0.86359 0.831232 0.781297 0.690707 0.49632 0 1 0.951325 0.904825 0.861447 0.820597 0.780023 0.735605 0.680579 0.603653 0.485464 0.294672 0 1 0.925029 0.854514 0.790472 0.732209 0.676734 0.619148 0.552595 0.467845 0.353178 0.197029 0 1 0.89487 0.798625 0.714813 0.642215 0.576728 0.512725 0.443697 0.362612 0.262776 0.140401 0 1 0.856437 0.731216 0.62905 0.546311 0.47642 0.412402 0.347751 0.27679 0.195308 0.101929 0 1 0.800218 0.641576 0.524855 0.438632 0.371294 0.313667 0.258907 0.202072 0.140084 0.0721134 0 1 0.703299 0.510659 0.390937 0.312902 0.257272 0.212804 0.172745 0.13295 0.0911057 0.0465165 0 1 0.502605 0.307234 0.215822 0.165285 0.132594 0.107986 0.086691 0.0661472 0.0450249 0.0228854 0 1 0 0 0 0 0 0 0 0 0 0 0

The potential looks like this: