This meeting in Shepard's quarters MUST be done before Priority: Rannoch, otherwise it will be missed and you cannot Lock In the Romance. She'll then suggest talking in private in the Captain's Cabin. When you get your first Dialogue Wheel, pick 'Do you still want me?'. Now you'll have a much more honest discussion, and during it, the topic of continuing your relationship will be brought up. Head up there and use the computer next to your Private Terminal to Invite her up. Shortly after she's revealed to be on the Admiralty Board, you'll have a chat with her in the Briefing Room, but she expresses that she would like to have a more candid conversation up in the Captain's Cabin. Fortunately, she has easily the quickest meet-to-Lock-In time in the game. Unlike every other Romance option in the game, Tali doesn't appear until past the halfway point, and before then it's unclear if she's even in the game! If you Romanced her in Mass Effect 2, you'll want to be patient and wait until she shows up in Priority: Geth Dreadnought.