I always get to a clear return with smaller enemas, usually 3qt, in knee-chest, before going for the 4qt enema.
I wait at least a good 45 minutes. Twice that if I had to take a Magnesium Citrate to get fully clear.
By the time I get to the 4qt itself, I usually use Simms Position with a pillow under my hips as well as one under my head.
I keep a clean dishpan next to the bed in case my system rebels and it starts coming out the other end. Just hang my head over, without getting out of position.
I take a lot of time.
I use a barium-type nozzle, on an open-top "great white" bag.
By then the mucous membrane is long ago washed away, so I need a SMALL AMOUNT of KY Jelly on the nozzle itself, take it SLOW and make SURE I have it pointed towards my navel. I always wait a few minutes, make sure I'm fully relaxed, make sure it's all the way in, and make sure there is not too much background stimulation.
Truth is it WILL hurt!! I make an effort NOT to think about that.