Ooohhhh I love enemas
I'm not a doctor but can give you my personal experience advice.
The safest way is to probably buy a kit from the pharmacy with something like saline. There are kits to connect to hot water bottles
I personally just use some surgical tubing and a zip lock bag with warm water.
If you use water make sure it's clean and safe, can always boil first. Luke warm is best afterwards, don't make it too hot you don't have pain receptors in there and won't feel it hurting you.
Gravity makes it leave the bag, the higher the bag the more pressure you will get, purge the line of air first, air doesn't go in as easy and blocks the flow. I don't recommend using shower or faucet attachments, too dangerous with the pressure.
Try a smaller amount like 250 mil first, then move up if it is comfortable.
Don't do it more than a couple times a month or you will flush out your beneficial bacteria.
It definitely makes me feel better in my guts after a good purge, plus it is a very satisfying way to mess your diaper if that's something you enjoy