I don't make jelly or wine, I only eat grape as it is, as a fresh fruit, when it's mature and sweet.
I think that climate, soil, pruning and cleanliness can affect the quality of the final product dramatically.
It's necessary to start new plants
when the old ones do not product fruits enough.
Unfortunately it's rather difficult for me to explain how to grow the grapes because I don't know English perfectly (my native language is Italian and I learned English at school many years ago). I can use only simple expressions and often I consult an English-Italian dictionary.
Just in these days I am reading an interesting book (De re rustica) that the latin writer Columella wrote 2000 years ago. He gives many advices about the cultivation of grapes.