The romance in the second story expansion for The Witcher 3: Wild Hunt is different than the romances available in the basic version of the game and in the first expansion. You won't find here a gradually built relation with Syanna that will end with happy end. Instead of that, this time the happy end is available immediately, without building suspense or spending nice moments on detailed conversations. In order to be able to romance with Syanna you must decide to help her during the main quest The Night of the Long Fangs (you can find the Hidden or find Syanna). You will get to a fairy world in which the girl was imprisoned.
During the Beyond Hill and Dale quest you won't have to try too hard to be liked by Syanna. You don't need to select the best dialogue options in order to obtain the ribbon from the girl with flints. The most important part of the conversations during this quest is when Geralt and Syanna go to the clouds (after the battle with the Cloud Giant). Here you must only agree on Syanna to "use you". The cutscene will trigger automatically.
Other love scenes in the Blood and Wine expansion pack
In addition romance with Syanna, in Blood and Wine expansion pack it is also possible to use the services of courtesans. In order to spend some nice moments with the girls, player must go to Beauclair, to San Sebastian and speak with madam of the building. Player can choose from three girls services of whose can be required one after another (the cost of a single service is 20 crowns). The girls will be marked on your minimap with a heart icon. The cutscenes are same as in previous games, you won't see any special scenes reserved for full-fledged romances.