How would I configure time capsule if I already had a wireless router with Verizon and only wanted to use time capsule for wireless files back?

Connect a permanent Ethernet cable from one of the LAN <--> ports on the Fios router to the WAN "O" port on the Time Capsule.

Use AirPort Utility on your Mac to set up the Time Capsule to create a new network.

Once the setup is complete, you can go back in using AirPort Utility to turn off the wireless function of the Time Capsule.

Backups will occur over the Fios wireless....or you can connect using an Ethernet cable to one of the LAN <--> ports to make things go 5-6 times faster.