She'll bring him somewhere far away where she's set up a fake threat and they're all alone, and then fake cry while she complains about being alone and lost. Steven will naturally want to console her, and in the moment she'll take advantage of his sincerity and naivety to fuse with him.
He might notice something is wrong upon fusing (Garnet didn't when fusing with Pearl), but by then it will be too late, Lapis won't be able to resist and will never let go. She'll fly them to some faraway planet on the pretense of hiding from the Gems for fear they'll imprison her again, while in reality it's because she knows he can't breathe in space.
He'll eventually succeed in his attempts to unfuse, but upon realizing he can't breathe in space (and Lapis hugging him, begging him not to die), they'll just re-fuse. She'll say she just needs a bit more time while she keeps him captive in the landscape of their minds, leaving them to live together in their own world.
After some time she'll start prodding him to go further, getting touchy/grabby with her hugs and rubbing cheeks with him, bringing their mouths close to kissing. He's afraid and hesitant at first, but he wants to get back home, so he goes along with it. He'll inevitably start enjoying it as it's his only form of contact with another person, and he'll become unable to live without her body.
Once he's hooked, she brings him back to Earth and lets him go, but only if he promises to keep quiet about it and if he'll live with her in the city. He acquiesces, telling himself it's for the best not to cause conflict between the Gems while he really just enjoys doing lewd things with her.
They feed the Gems some story about being kidnapped by HW and being kept in stasis, barely making it out alive because of a ship malfunction. He says he wants to live on his own now due to some things he found out, but doesn't mention what they are, leaving the Gems to wonder what past atrocity of theirs he discovered.