Q: airport time capsule without internet

Do not connect an Ethernet cable from the modem to the Time Capsule.

Set up the Time Capsule normally to create a wireless network...and ignore the warnings about not having an Internet connection or Ethernet cable connected to the Time Capsule.

The downside to this is that you must manually switch from your normal wireless network to the Time Capsule wireless network when you want to back up....then manually switch back to your normal network once the backup is done.