OK, let's go one step at a time and get the Time Capsule set up and operating correctly.....then, we'll tackle the AirPort Express.
Power up the Time Capsule for a few minutes
Hold in the reset button on the back of the Time Capsule for 7 to 8 seconds, then release
Allow a full minute for the Time Capsule to restart to a slow, blinking amber light status
Connect an Ethernet cable from one of the three LAN <--> ports on the AirPort Extreme to the WAN "O" port on the Time Capsule
Click the WiFi menu at the top of the Mac's screen
Look for a listing of New AirPort Base Station
Just below that, click on Time Capsule
The example below shows an AirPort Extreme as the New Base Station.....you will see Time Capsule on your screen

If you want to use an iOS device for the setup instead of the Mac.....
Go to the Home screen
Tap on Settings
Tap on WiFi
Look for Set Up New Base Station
Tap on Time Capsule (The example below shows an AirPort Express, you will see Time Capsule

When you click or tap on Time Capsule, that will get the setup "wizard" up and running. It will take a few seconds to analyze the network, then suggest the correct setup. The screen will look similar to the illustration below, except that you will see your devices and network name.

Type in a name that you want to call the Time Capsule. You can use the same device name that you used before
Click Next
The setup wizard will set everything up for you automatically. Watch the next screen very carefully to confirm that that the Time Capsule is being set up to "extend using Ethernet", which is another way to describe a roaming network.

When you see the message of Setup Complete, click Done. That's it.
The Time Capsule will now use the wireless network password for both the wireless and as a device password. If you want to assign a different password for the device password, you can do that later by changing the Base Station password setting in AirPort Utility.
It may appear as if the Time Capsule is going to make a new backup of the Mac, but it should recognize the old backup after awhile and adjust.