How do I know when to cut back my clematis for the winter?

See if this helps!

https://www.gardeningknowhow.com/ornamental/vines/clematis/pruning-clematis-vines.htm

some clematis bloom on new growth and some on the old growth that looks like its dead, but it is isnt..if you got ambitious last year and did a real good trim chances are you cut off some of the old growth hence less blooms...learn what variety you have before any pruning is done.

Barb, if you know what variety your clematis is, google it to see when the best time is to prune it. If not, I would do the opposite of what you did last year. So if you cut it off last year, leave it alone this year. Also next spring apply some slow release fertilizer like Osmocote around the base of the plant. Good luck!