Hello! What you are going to have to do is install windows then go into disk management and partition a new simple volume. Once you have done this you will insert your boot USB drive and install OSX on the new simple volume you just created (After you erase it ofc) Then once you are in OSX after everything is configured you need to restart it, download multibeast or chameleon. (Your choice) And select a theme. Then at boot you will have the option to choose between your OSX partion or your windows one. Let me know if you need any further help!