I am trying to understand a counter-example to Mostow's rigidity theorem.

Here is the counter example I want to understand.

Take two non-isometric octagons with the sum of interior angles equal to $2\pi$. Then they form fundamental domains for non-isometric surfaces.

My questions are: how can we visualise such two hyperbolic octagons, how can we see that the fundamental groups of these surfaces would be isomorphic?