Block in the correct colours in the correct places. Get the basic shadings right to show the curling nature of the stems. Preserve the roundness also.
In painting the grape, don't be too concerned if you overpaint your grape into the stem. That way you don't have to make your stem too wide to fill in any gaps.
Be sure to get the shadings correct where the stem connects to the grape.
If a stem gets lost against the background, leave it that way – it's natural.