How to arrange 8-to-1 multiplexer to form a 64-to-1 multiplexer???
Use nine 8-to-1 multiplexer?? just want 1 output...

 

Arrange 8 8-to-1 multiplexers side-by-side, to give you your 64 inputs, and collect the outputs from these multipluxers and feed them to the inputs of the 9th 8-to-1 multiplexer, to give you your 1 output.

Dave