Cutting windows into a premade gingerbread
Very, very carefully.

Although that's rather tongue in cheek, it's true: slowly and carefully is the key approach. The quicker you try to move, the rougher the shape you produce. You also significantly increase the chance of breakage.

Make an initial rough hole at the center point of your eventual window area, and then cut out to finish the edges. A serrated knife used with a slow, steady sawing motion should work.

Luckily, if you do end up with lopsided, misshapen, or unexpected results, icing and candy decoration can help hide the mistakes :)