If you are planting a new Clematis, you can use the table below which sets out when to prune each group of Clematis. There is an old gardening saying, which might help if you are not sure what type of Clematis you have, "If it flowers before June, don't prune."
Clematis which flower early in the year, March, April, or early May are mostly likely to be of the groups known as C. alpina, macropetala and montana which for pruning purposes are 'Group 1' Their flowers are produced on growth made the previous year, which means the plant must not be pruned early in the year in the spring, or you will cut off the buds and branches on which the flowers are formed, and cut off the potential flowers. This group does not require pruning; but can be pruned after flowering if they are outgrowing their space.
The next group of Clematis, very popular because they have large showy blooms and flower from Midsummer such as 'Nelly Moser', (Image above center) 'The President Niobe' and many other varieties are all known as "Group 2"
Finally, the late flowering Clematis are a bit confusing, some have large flowers such as 'Jackmanii', others can have small flowers such as C. viticella, so the flowers are different but distinguishable by the fact they all flower late in the season and are known as "Group 3"
If you make a note when your Clematis flowers you can make a judgement on how to prune it: early spring flowering are usually group 1; summer flowering group 2 and later summer flowering group 3. This is is a broad brush approach as the only fail-safe way is to correctly identify the Clematis, and then to check to which pruning group to which it belongs. There are hundreds of types of clematis and new ones being bred all the time so identification is not always easy. Now you know which Clematis or pruning group it is - what to do next ?