Practically always lie in bed on your left side, left leg straight, right - slightly bent at the knee. When most of the solution has been injected, you can straighten both legs - it will become easier. When all the enema is in the intestines, you can kneel and lift your bottom up. The solution will go deeper and your wife will also like the view from behind.
After some 40 years, my go-to position to begin an enema is still what a home health visiting nurse taught me — lying on my left side, left leg flexed — what’s shown in the old fashioned nursing manuals. That’s the most comfortable for me when I am deep breathing and inserting the tube myself or when my husband has “the honors.”However, I might shift positions during the enema, depending how I am feeling. If I’m getting too crampy, I’ll stop the flow and stretch out my legs. If I need to retain the enema longer because I was bound up, I am still limber enough that I can get onto my knees with boobs down, bottom up. That does wonders to relieve the cramps and I can hold the solution longer. Also helps getting the enema higher into the transverse portion of my lower intestine. My husband has always enjoyed me in this position. LOL