GA / Evolution noobee question

Say I have hundred NNs of which 90 perform badly and ten perform "ok" to play a game. Now how do I seed new NNs from these ten? What "makes" a particular NN is its weights. So how do I now generate new NNs from these ten with random mutations?

How do I even know that there is something like an "optimal" NN that can actually play the game? With a math background, I think there must be something like …

evolution neuralnetworks