This is super-simple.

• Only an 11 x 11 grid
• Each 'plate' of the capacitor is only 3 cells wide
• In 2-D Laplace's equation, each cell is the average of the ones around it.
• Cell B2 would be typed in as =(b1+b3+a2+c2)/4
• This cell is then copied over an 11 x 11 grid, first  B2..L2 and then down to B12..L12
• (If it complains about circular references, press Cancel, and go on.)
• The cells outside of this region have zero in them so we may think of them as grounded
• We put the capacitor in the middle of this region.
• Cells F5..H5 are set to 1000. This is the positive plate of the capacitor
• Cells F9..H9 are set to 0. This is the other plate of the capacitor
• Under Tools/Options/Calculation
• Select Iterations, and set it for 100 iterations.  Select Manual recalculation
• When you press F9 it will iterate until it gets within 0.001 of the previous value in each cell, (or until 100 iterations are completed)
• The results should be left-right symmetric.
• Block the 11 x 11 grid with the mouse.  Go to the chart wizard and select Surface plot.
• Work through the chart wizard steps (labeling axes, etc.) to finish the plot.
• The electric field is the negative gradient of the potential, so one gets an idea of the electric field inside and outside the capacitor. (One wants a bigger grid for this to have much meaning)
• The surface charge density is related to the surface electric field so one can see a greater charge density between the plates (on the inside of each plate) than outside (the outside surface of each plate).