T rings are made for a particular camera. The T ring goes between the end of the relay lens opposite the eyepiece and the front of the camera body where the lens normally would attach. Crack the manual on the thing or call the manufacturer for another copy. Experiment ahead of time. Go back and forth between everything that will focus, peaking everything repeatedly for the best overall picture.
Relay lenses can be expensive - two to five hundred dollars.
T rings usually run about twenty bucks. Save the box it comes in so you can get another one if you lose or break yours. T rings can be purchased at most decent camera stores. If you need another one, take your camera body and relay lens to the store so they can sell you the right one. One popular brand of night vision uses a simple and inexpensive adapter that lets your existing 50mm camera lens serve as a relay lens.