How can one, as a man, help make this happen?
I don't know, I have never done it myself.
I can tell you how my girlfriend does it. Of course she's a woman, but maybe it will be useful somehow, and if not useful, then at least interesting to read:
1. She gets to know the girl first really well and observes her. She doesn't want sour atmosphere - neither during the "lesbian opening" process, neither later in the form of regrets. She avoids too conservative girls and also emotionally, mentally unstable ones. The optimal target is a confident hedonistic girl, who had many men and is generally "relaxed" when it comes to sexual matters. But a "normal" girl is ok also.
2. She invites the other girl (alone) to her place for talking, gossip. "Let's hang out and talk at my place."
3. Offers wine or other alcohol. If the other girl prefers coffee, she will offer her alcohol again later.
4. Starts talking about life, men, sex, etc.
5. Stars to act giggly and sexual. Starts look the other deep in the eyes. Bite her own lips and giggle. Comment on how sweet the other girl is, how the other's lips looks sweet... She can also compliment eyes, hair... She can hug the other girl and say "you're so lovely! I love you so much".
6. She can kiss the girl on a cheek in a "friendly" manner... And proceed with a closed lips kiss on the lips. Then "Wow, your lips are so lovely, you're making me crazy!".
If the other girl reacts well, she proceeds with a normal kiss. She tries to make it long and tender, delicate.
7. At this moment she likes to do "sweet vibe" phase. She doesn't proceed further, but instead holds the process at "we just kissed and it's lovely", continues light cuddling.
If the other girl has any regrets, my girlfriend comforts her and tell her it's ok. She tells that things like "You're so lovely... Don't be silly, it's was just a fun kiss, nothing to worry about". "Have you never kissed a girl while drunk? Really?". "It's just fun, relax....". "Nobody sees us, don't worry. Why so serious? (giggle)".
If the mood is too difficult, she will stop the lesbian seduction mission and change it to normal girl-girl talk.
Often a girl will feel weird, but after a while will get over it.
8. If everything is fine, she can play with the others hair, kiss her neck, kiss her ear or kiss her again on the lips.
Her goal is to keep it light and sweet, fun, but at the same time "covertly" make the other girl strongly turned on.
When the other girl is "cooked", she ask her to massage her (GF's) breasts. "You have such lovely, gentle hands...." She can kiss her hands, and place them on herself. She says the other girl often kisses her breasts out of her own initiative.

This or the other way, my gf then starts working on the others breasts. Or she could start with them, but she says placing the others hands on her own breasts is somehow more cool.
9. If everything is proceeding smoothly, then of course the matters go down to clitoris massage, then licking and fingering.
Few things:
- The whole process isn't illogical at all, she knows exactly what she's doing and why. The "women are purely emotional and don't understand seduction" stuff is nonsense.
- She puts huge emphasis on the vibe. Whole thing must be cute and sweet, only then sexual. The other girl's comfort if extremely important. If there is friction, my GF immediately retreats and comforts the other girl. Later she can try it again or just abandon the idea for the evening.
- She frames it as "fun", not serious sexuality. "We're just girls having fun" But she never goes too crazy and always keeps being caring and supportive.
- She frames it as something that cool girls do to have fun. As something both normal and "high value". Like accepting her advances will make the other girl more cool, move her into better category of girls who know how to have fun. "Wow, you really have never kissed a girl before?" - "You're so sweet and innocent". There is no pressure there, and she is super warm and nice, but at the same time she does send the message that the other girl could become more cool and "sophisticated" by doing lesbian stuff with her.
- She makes no "smart" speeches about repressed desires. She thinks that would be too heavy and weird and "WTF". She keeps her language simple and positive.
- She pushes the situation bit by bit - it's a "small steps" method.
- She says it's often EASY and people just think it's hard. I think in her case, her personal traits play a big role (great looks, great kisser, confident and relaxed, etc), but she's also right about the whole thing being much easier than most people think.
- She says that maybe 1/3 of girls can be easily converted to bisexuality. The rest are too conservative and stressed - not necessarily not bisexual.
