Establish that your hydrangea is healthy enough to take a pruning. If diseases are present, take care to treat those diseases prior to any pruning. If you prune a diseased plant, then you run the risk of spreading the disease.
Sharpen the pruners to avoid any shredding of healthy hydrangea stalks. Sharp pruners will make the pruning job much easier.
Prune a Nikko Blue hydrangea to the first pair of new growth buds, under the old flower bud, from the previous season. Locate the stalk that the flower had previously bloomed on, and cut to the new growth of buds on that stalk.
Remove any old wood that might be on the hydrangea during the pruning process. Cutting old wood encourages a nicely shaped plant.