Clematis flowering in spring — nothing grows at the bottom on old wood, but only flowers at the end of stems that are over the wall
out of site Sometimes gets second lot of flowers late summer. How do I cut back for next year?
It sounds as though your clematis blooms on new wood; therefore regular pruning which stimulates new growth will produce the most flowers. Here is an article on how to prune clematis. https://www.gardeningknowhow.com/ornamental/vines/clematis/pruning-clematis-vines.htm
You may also find this article helpful: https://www.rhs.org.uk/advice/profile?PID=344