CAUTION: Failing to pursue this secondary quest and Now or Never not only prevents a romance with Triss from happening but also seals the fate of all mages in Novigrad.
Triss asks you to help her with a task that leads you both to a Masquerade ball of a noble woman who will help Triss in her efforts to get mages out of Novigrad if she helps her get her son Albert out as well.
There are several little things that happen during this quest that can help you romance Triss.
BROWNIE POINTS #1: When she asks you to go buy her mask, buy a new set of fancy clothes as well and wear them when you go to meet her before going to the ball.
BROWNIE POINTS #2: When Triss falls into Geralt's arms in the Garden KISS HER. If you don't than there's no chance she will stay in Novigrad with you at the end of Now or Never.
After the kiss Albert informs you that they need to leave immediately and forcing Geralt and Albert to switch masks. Geralt must lure assassins while Triss and Albert getaway. After fighting them in the garden you can meet up with them at the barn. Triss will try and apologize if you kissed, tell her she doesn't need to apologize.
When the quest is over the final stage of Triss's plan is available in the form of the quest Now or Never. This is also the quest in which you have the chance to romance Triss.