It if flowers before June don't prune
Things that flower before June are flowering on wood or stems produced last year so don't cut them off before its had chance to flower, if you have time to prune it properly. Plants that flower later can have a trim in the early spring because they will flower on new growth later in the year.
Whilst this is the rule the reality is its better to prune almost anytime, when we have time than never. Its a rule we break when we cut evergreen foliage for our wreathes removing all the red foliage from our winter evergreens photinia and marbled leaf from our laural. There is nothing worse than an over grown woody shrub that needs a good chop.
If like me you struggle with an overgrown phipadelphus that should be pruned after flowering and usually gets forgotton you will no doubt of lopped it down to size perhaps in the spring just because its such a mess. and probably had very little or no flower to follow. The plant will recover in the following year. The best senario is to reduce it by a third this year and the following years respectively but that is a bit slow. Whilst I will painstaking reduce my tall shrubs slowly using the correct cuts and careful observation my husband unfortnately has a hedge timmer he likes to use it, acheiveing instant reduction in unruly plants.
To prune you need a good sharp pair of secateurs. It you are removing thick branches you may need long handled loppers or a pruning saw. If you are pruning climbers you may need some wire ,string or foam wrapped wire to tied in the stems or branches to supports
Extend-able loppers and useful slow release plant feeds for the spring as plants begin to grow.
Pruning can do a number of good things. Its particularly good for improve the shape Its also good at encouraging fresh juvenile colourful shoots or foliage as with Cornus,(dog wood) and photinia (Red Robin). When a plant like a Wisteria is overloaded with flowers and whippy over long branches is a useful way of reducing the load on the plant to 5 buds per stem instead of 15 and ensuring they all make great flowers rather than straggly weak one.
A simplified Explanation of Pruning
Pruning can become unnecessarily complicated. So remember all you really want is your plants to make nice shape and remove any poor or damaged growth and with a number of shrubs like type 3 clematis,buddleja and lavatera to reduced the hight so this years growth does not put the flowers in the sky.
Most pruning makes the plant stronger and helps it put energy into growing stronger stems and more branches. As long as you don't give it a hair cut just before the winter frosts so new growth is tender and burns the worst you will probably do is cut off this years flowers.
Plants produce more flowers on lateral horizontal branches. Taking out the leading point will encourage more growth to the side and this growth should produce more flowers.
Keep your tools sharp and clean that means dip them in disinfectant because to can spread viruses and fungus from plant to plant when pruning sick branches off your plants.
Don't cut too far away from a bud because you will end up with a stub of ugly dead wood and the half a cm will naturally seal itself the 2cm will just go brown and look untidy.
When you prune do trim close to a bud. If there are two parallel buds as on a buddleja cut straight across slightly above them. When there is one bud just below where you want to cut check it is growing outwards and not into the shrub. This bud will become a branch and you do not want crossing branches. Cut on an angle away from the bud. A slight slant away so water does not run into the bud and cause problems.
Before you prune the most important think to consider is when does the plant flower. Remember if it flowers before June don't prune yet wait until its flowered then prune it. Most plants thrive so much better from regular pruning.