Hi Barbara, You don't say where you live, but down here in Cornwall we have lots of Hydrangeas as they do very well here! The problem with them is that they form next years flowers very early, (infact often in the late summer of the previous year) and hold them in the terminal buds overwinter, so, if you prune and cut these off you won't get many flowers the next year. This means you have 2 main ways of pruning, the first is to prune before next years flower buds are formed but this means doing it before this years flowers are really finished, about mid-August. The other and more common way is to remove 1/5th of the bush down to 20cms in winter, this leaves 4/5ths to flower and the bit pruned to grow new growth from the base, and over a period of 5yrs you will prune the whole plant. Given the spectacular late display this year from the Hydrangeas given our fantastic weather in September and October unless the plants are very big and in desperate need of pruning I would advocate the latter method. Finally I would say that even if you do make a mistake all that will happen is the loss of that years flowers.
best wishes Lannerman.