Water-based lube is the best thing to use. That means you can use condoms, gloves and/or toys without problem. Look for something that states it's for anal sex.
Forget the enema. It could be unpleasant to have water squirted into your rectum, though some like it. It's better just to accept that whatever goes in might have a little shit on it, and that's normal.
Talk is important: communicate with your partner. Make sure she's feeling sexy and aroused, with lots of foreplay on her first that goes nowhere near the ass. The best position is missionary with her on her back and legs up so you can keep eye contact and kiss. It's not as easy as doggy style, but it helps your partner.
With toys, don't use anything that's not designed to go up the ass. A flared base ensures the toy won't be pulled inside. The sphincter muscles are strong and can squeeze involuntarily, and once the toy's completely inside it's difficult to get out.
Rubbing lube onto the outside of the asshole will prepare it. Then you can slow slide one finger inside, with lube on it. There are two rings of tight muscles so ease them open rather than push in. The doctor's trick is to ask a patient to breathe in deeply, then breathe out slowly. The breath out relaxes muscles, so you move during that, slowly and gently. Anal beads can be a good toy to try, as the beads get bigger you work up to them. Don't expect too much on the first session. Wait at least two days until you try again. Give her absolute control of when you stop, how deep and how fast, and tell her that what she says goes. That builds trust, and, if she trusts you, you will be allowed more. Some people just hate it, so respect that. The more gentle and loving you can be, the better the chance she'll begin enjoying it. During, tell her she's beautiful and mean it. Use the breath trick for withdrawal, as a sudden pop ain't fun.
If you really want to build trust and an equal relationship, offer her your ass. It won't turn you gay, and you might enjoy it.