Journey from the British meuseum to Apollo victoria theatre ?

I am travelling to London to visit the British London museum, I then need to know what station to go to and what train to take to get to the Apollo Victoria theatre. I don't live in London I will be travelling from Milton Keynes so vaguely know the stations apart from Euston so please advise clearly as if explaining to an alien or child. Or if you know any day tickets to use various trains and underground would be good. Thanks