Thin the clusters of grapes as they appear, while the flowers are still falling to the ground. Pinch off groups of misshapen grapes or of fruit that are too small or too large for that stage of development. Compare with the majority of grape clusters on the vine to see what the normal size is.
Thin the fruit on the retained grape bunches by cutting and discarding the lower half of each cluster's central stem.
Prune your seedless grapevine according to additional specific recommendations for the cultivar you have. Contact your local cooperative extension office or nursery if you need to obtain care information on the grape variety you're cultivating.
Remove diseased and broken canes as you notice them. Make your pruning cut ¼ inch above a healthy bud at a 45-degree angle that slants upward.