How to set up Time Capsule in wireless bridge mode

I can't find any clear, step-by-step instructions on how to set up a Time Capsule in wireless bridge mode. Eg, how to disable its routing functions and set it up as a switch/repeater connected wirelessly (not via ethernet cable) to another wireless network. I'd also like to be able to connect a backup harddrive to it via USB and have access to it on the wireless network. Anyone know exactly how to do this?

In AirPort Utility I change the default setting from Router to Bridge Mode, but then it's not clear how to specify the existing network I want it to connect with and extend. Do I use "Create New Network", or "Add to Existing Network"? And how? Honestly I've tried multiple variants of both and neither seems to work right.

"Add to Existing Network" seems to make my Time Capsule completely vanish and and AirPort Utility can't find it. "Create a New Network" at least doesn't do that, but I can't tell whether I've successfully extended my other network with my TimeCapsule or not, and the USB harddrive is not available (nor TC's own internal 3TB harddrive for that matter). For example, there's this comment on Amazon saying you should use "Create a New Network" instead of "Add to Existing Network".

Anyway, any suggestions, or links to a good guide on this, much appreciated.