Trance is a natural phenomenon, and we go in and out of trance every day. The difference between natural trance and hypnotic trance is simple: with hypnotic trance, you're intending to do it.
One way you can practice putting yourself into trance is to set a timer for 2 or 3 minutes, and then set an intention for the session. A good intention for a self-hypnosis session would be to experience your Happy Safe Place, or repeat a few suggestions around an outcome you want to create. Start the timer, and then use awareness of your breath to bring yourself into an internally-focused state, focusing on whatever your intention is for the session. Bring yourself out when the timer goes off.
Personally, I even play with my subconscious's amazing sense of time! You can start the trance session by saying, "and, in 3 minutes, I will emerge just a few moments before the timer goes off". Then see what happens!