Most reliable way to sign up is go the Wines of the Old and New World tasting (included) on the morning of the first sea day which will be listed in Passages (sometimes a side picture ad). This is where you can taste many of the included wines and meet the sommeliers. They will have a sign up for the wine lunches or dinners at this event. The wine tasting is a great way for the sommeliers to get to know you and learn your preferences. I only like dry wines, so they are used to my asking at dinner about what I would like. You can ask for any of the included wines in any venue. They may have to go get it from somewhere, so you might need to be patient. They generally have a red and a white that is recommended at each meal, but if they aren't to your taste, just ask for something else. I seem to find a couple whites and a couple reds as my "fallbacks" for the cruise within a couple days.
On our Splendor TA in April, the Sommeliers were taking sign up on embarkation day in La Veranda during lunch. We only had one on the TA and it was a champagne tasting lunch.
Or ask the sommelier in one of the restaurants. Or call reception and have them leave a message for the head sommelier.
I think they are a GREAT way to use OBC!