Best way to make wireless file server with spare RT-AC68U?

I have two RT-AC68U routers, one of which is my broadband router, while the other is sitting unplugged in a closet. (The latter's broadband port was killed by a lightning strike, and while I eventually figured out how to repurpose another port for broadband, I'd since purchased a replacement, which BTW is a different version with a significantly faster UI. Except for the broadband port, the damaged router works fine.) I'd like to use the idle one to make a wireless file server. I can use...

Best way to make wireless file server with spare RT-AC68U?