I don't have a ritual and cannot remember ever having an enema administered when i was a kid.
my introduction to enema was when i was 7 or 8 and a neighborhood boy friend (who i had a crush on), showed me His enema when i was at His house and explained how His mom would "stick it up [his] butt." Even at that young age, He was pretty sophisticated and manipulative, almost like He was grooming me? Or maybe that was just wishful feeling? We were young. But His explanation definitely stirred excitement and an awkward feeling in me. When i went home, i found our enema kit in the top of the bathroom cabinet. It had a red bag, a long tan tube, a black screw in plug with a hole in it to connect the hose to the bag, and a cream colored, rigid nozzle. It was bigger at the insert end and had four indentations with two holes each at the bulb end, then tapered down to where it connected to the hose. i focused on the nozzle and it became my first dildo and the first thing i ever put in my own rectum.
To this day, the idea of getting an enema from someone else evokes the same feeling i had at age 7, awkward excitement.
In another discussion on enema, a Dom woman really nailed it for me (even though i'm gay). For me the psychological aspect/s are surprise and affectionate control. She talked of waking Her sub up by "pulling his panties down." She had me there. i am not feminine and don't wear panties, so there would be a real mind fuck aspect for me to wear panties to please a partner, definitely a subjugating aspect. She next talked of him waking to feeling his "ass cheeks spread," or feeling his "hole violated."
All of those aspects: being clothed in something that makes me feel awkward in the first place, then having that manipulated further by being stripped and awaking to that, being penetrated with an enema nozzle and waking up to that. Those are all HUGE mind fuck turn ons for me.