To connect a t-ring to a Celestron Regal M2 zoom eyepiece, you unscrew the twist eye cup anti-clockwise until it comes off. The attach the adaptor ring to the thread, followed by the t-ring for your camera. Note the t-ring is available separate, and the adaptor ring is supplied when you buy the spotting scope (not included with the eyepiece when bought on its own)
You can then enter the wonderful world of digiscoping. Note that with some cameras you will only be able to use manual exposure. Use live screen for more accurate focus. See video below for a hands on demonstration.