I find that the best way of introducing a new GF to the joys of giving and receiving, is by NOT trying to start off with an enema just for the sake of an enema.
There must be trust.
I think the best way to introduce enemas into your relationship is by first creating a situation involving analplay during foreplay and sex. That then gives you an excuse, to want to be clean inside your anus, for them to finger whenever they want.
You can say "That was really nice when you fingered my anus, but I'm embarrased that there may be some shit inside me". "Maybe you should give me a suppository or an enema to wash me out first so that my arse is always clean for you".
Then go shopping toghether, but start out small, eg a squeeze bulb enema or Higginson hose with the inline bulb and nozzle, for her to use on you first up. Your objective is to use this on her arse as soon as she is comfortable using it in you. So don't dare reveal your own 3 litre can with the big dildo nozz you may already have hidden up in the back of your wardrobe. That comes later.
Everyone is different and we are rarely the first lovers, and usually not the last. This is just one example. Maybe it will work for you. And it is not gender specific, so it's the same whether you are a Girl or a Guy.
So, based on her feedback, after she has given you an enema and fingered your nice clean arse to her heart's desire, and if she now trusts you sufficiently, then you can suggest giving her an enema too.
I am male and I was originally introduced to giving and receiving by a young skinny GF way back in 1973. The very first time we had sex together, she started fingering my arse while I was cumming. This really felt good when she hit my prostate. But later she said she had to shift a big turd out of the way to get her fingers in, and this really embarassed me.
Then we went for a walk to the local shopping centre and she bought a big red German douche syringe with the long thick black smooth ebonite nozzle, which used to be plentiful in Australia in any Chemist Shop in the 70's - but long since disappeared.
When we returned to my flat, she filled my bathroom wash-basin with hot water, and sloshed a bar of soap in circles to make it all suddsy. Then, she bent me over the bath, greased up my anus with KY jelly, and proceeded to pump the entire soapy contents of my wash-basin up my anus, one long squeeze at a time. Then fill it up, lube the anus, stick the the big black nozzle in and squeeze, then fill it up, lube the anus, stick the big black nozzle in and squeeze, until the basin was empty and the nozzle just slurped air.
We lived together for another three years after that, until our ambitions led us in different directions.
I just love it when a girl gives me an enema.
Cheers,
Brachio