Time Capsule setup with a TP-LINK TD-W8968
How to use a 3TB Time Capsule with a TP-LINK TD W8968 ADSL2 modem/router?
MacBook Pro (17-inch Early 2011), OS X Yosemite (10.10.4), 2.3GHz / 16GB / 1TB SSD
The primary uses of the AirPort Time Capsule are to serve as a backup hard drive for your Macs and as a router to provide a wireless and wired network signal for your devices.
If the TP-Link modem/router is already providing a wireless network signal, and you don't need another wireless network for guests or something similar, then you do have the option to turn off the wireless on the Time Capsule and use it simply as a backup drive once the basic setup of the Time Capsule is complete.
If you are asking how to set up and configure a Time Capsule, that process is determined by whether you have a new Time Capsule that has not ever been set up yet, or whether you already have a Time Capsule that you have been using on your network.
If the Time Capsule is new, you can go ahead and connect an Ethernet cable from one of the LAN <--> ports on the TP-Link modem/router to the WAN "O" port on the Time Capsule and then use the setup "wizard" in AirPort Utility to set up the Time Capsule.
If the Time Capsule has been used in the past, then you will need to perform a Hard Reset by holding in the reset button on the back of the Time Capsule for 7-8 seconds and then release the button and allow a full minute for the Time Capsule to restart back to a slow blinking amber light status. That is your signal that the Time Capsule has been reset correctly and is now ready to be hooked up to the TP-Link modem/router as above, and then set up to work with your network.
The Time Capsule Setup Guide, which came with your Time Capsule provides basic information on how to setup the Time Capsule along with some general troubleshooting tips. If you do not have that guide handy, a link for the Setup Guide is here: