Fish tape will work well. Cut whatever size hole will allow you enough room to work; if you have textured walls, cut as big of a hole as you can... Textured walls are so easy to patch it isn't even funny. If your walls are not textured, then be much more careful on how big you cut as they are not as forgiving.
Good luck with ripping up floorboards.
Sorny
Good luck with ripping up floorboards.
Sorny