Be as drastic as you dare! Cut all of last years stems down to approx 60-90cms from ground level. The new shoots that develop will carry the flowers for this summer. If you cut at slightly different levels this will encourage flowers at different heights; You can also cut down dogwood close to base to encourage new growth.
If your a bit nervous about pruning you can start with the obvious to give you a bit of confidence.
1, Any dead or dying, wood cut back to a healthy bud (remember to cut above bud at the angle you you want the bud to grow -outwards)
2, If you spot any diseased wood or leaves remove immediately to avoid further spread.
3, Any wood that has been bruised, broken or damaged in any way , cut it out.
4, Any wood that is crossing another or rubbing up against another , take this awat as it may cause damage /disease.
5, Keep your secateurs SHARP!
Happy Chopping !!