Hello, I just purchased a few potted grapevines that are healthy and thriving. They are fully leaved, trying to produce grape clusters, and have 3 to 4 vines each on them roughly 4 feet long on each plant. I am getting ready to plant them, and want to train them to a hog panel trellis using the using the Four-arm Kniffin system. 2 are Mars Variety, one is a Candice, and one is a Lakemont. From what I am reading, all varieties are best using the spur pruning method (which I'm still not sure I fully understand). My question is, I know that I need to trim all vines but one to create the trunk. Is it too late to do this? Do I do that now or wait until winter or next spring? And also, what exactly do I need to do to get the main vine to grow the 4 arms to create the Four-arm Kniffin system? Thanks So Much! Mandy