You want to do what is known as a retention enema. So ideally not on a loaded rectum. If you are worried that the rectim is loaded you can perform a normal enema first.
With the retention enema you do not want to pass it, and you want to use small abouts of Luke warm water. Maybe 50ml.
I would personally use a syringe and tube, or a empty phosphate enema bottle from the pharmacy. ( you most definitely do not want to retain a phosphate enema)
Once in lay on you left hand side with bun slightly up to help gravity assist.