The wine was already done fermenting and tasted flat, I believe. THEN you can take a glass, dip the end of a damp toothpick in the acid of your choice and swirl it in the glass. We usually just put a couple TBLS in the glass when doing this bench trial. Put different acids in different glasses to see what tastes good. It's amazing what a bit of acid will do to "brighten" a flat wine. We did this test at a wine club meeting as some had never experienced the different acids in a flat wine. They were quite surprised at the difference it could make!
How old is this wine? That makes a difference too. Wine goes through several phases and it's good to taste all along the way so you understand what is "normal". If it hasn't been sweetened yet, do a little test in a glass with that as well. Even wines that you want "dry" tasting, can be helped with just a touch of sugar to smooth it all out.
As you can see, there is no wrong way. It's just whatever you like the taste of.
Have Fun!
Debbie