There are hardy and non-hardy geraniums.

For hardy Geraniums use seeds - the brown seed heads where the flowers grew.

For non-hardy ones, take a side shoot that has no flowers, or at least take the flower stalk off. I always root them in water as they readily root in a week or so. I would take non-hardy cuttings now as long as long as you keep them inside over winter.

Take cuttings about 3 - inches long. Choose a healthy shoot and cut it from above a leaf joint. Remove all lower leaves leaving just two or three. If there are too many leaves moisture will be lost and the cutting will wilt and die, none or not enough and it won't photosynthesise.

Pop the cuttings into potting compost mixed with fine grit, put them in a warm slightly shaded position and water the cuttings very sparingly keeping the compost barely moist.