Pretty common sense how to get there. During her side quests, I took Judy's side during major decisions and didn't backstab her/betray her trust and I responded to her texts. You'll know you're on the right track because later in the story, she will send you
texts and ask you to crash on her couch when you have a relic issue (I told her about dying from the relic/Johnny's engram) and Johnny even calls you out at one point about feelings for Judy (which is another point...Johnny constantly tries to get you to do something the opposite of what Judy wants, so I always ignored him for that). During the penthouse mission, I stuck to Judy's plan and I used non-lethal on the person who tried to turn the plan to their advantage and become a new crime boss. Judy kisses you on the cheek and leaves after the mission. If you made the right decisions, then you get a final quest where Judy asks you to meet her near a dam. When you get there, take her up on her offer. What is funny is she developed some sync BD tech that actually lets her hear bits of Johnny Silverhand when she links with you (presumably knowing about that because I told her). You get a cool scuba diving bit where you visit a sunken town and learn more about her, then after she asks if you want to stay the night at the cabin and I picked yes. You later find her in the bathroom upset because she finds out her Clouds plan went to shit and a bunch of people got killed as revenge and the Clouds is now shutdown for the foreseeable future. Do the touch thing to comfort, then you get the sex scene. The next day, when she asks you what it was, if you tell her the start of something good, then you unlock the relationship with her and she gives you keys to her apartment, which you can visit whenever you want now and shows up on the map as a new apartment base for you. That is as far as I got thus far with her, so not sure how much this may impact the ending or if there will be any other quests..