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 0.975943075 0.952771726 0.930827394 0.909723069 0.888161005 0.863590479 0.831232445 0.781296893 0.690706835 0.496320088 0
1 0.951324892 0.904824732 0.86144716 0.820596865 0.780022644 0.735605088 0.680578966 0.603652755 0.485464236 0.294671829 0
1 0.92502865 0.854513854 0.790472235 0.732208534 0.676733865 0.619147933 0.552594964 0.467844713 0.35317822 0.197028798 0
1 0.894869717 0.798624914 0.714812972 0.6422154 0.576727727 0.512724906 0.44369746 0.36261221 0.262775664 0.140401016 0
1 0.856436885 0.731216217 0.62904986 0.546311352 0.476419789 0.412401559 0.347750922 0.276790239 0.195307991 0.10192865 0
1 0.800217624 0.641575731 0.524854798 0.438632399 0.37129369 0.313667069 0.258906628 0.202071854 0.140084369 0.072113381 0
1 0.703298924 0.510659143 0.390937467 0.312902083 0.257272023 0.21280404 0.172745133 0.132950452 0.091105658 0.046516498 0
1 0.502604965 0.307233983 0.215821552 0.165285467 0.132594194 0.107986001 0.08669098 0.066147184 0.045024949 0.022885362 0
1 0 0 0 0 0 0 0 0 0 0 0

The potential looks like this: