Spread aconites around after flowering, or plant very fresh bulbs, patiently wait for them to spread themselves by seed, or sow seed yourself.

Do winter aconites spread?

deep from the base of the tuber to the surface of the soil. Winter aconite is a small plant, no more than 4 inches (10 cm.) across for most plants, so don't worry about crowding them out in the garden bed. ... As long as you have planted the tubers in fertile, well-drained soil, they will grow and spread year after year.

Is winter aconite invasive?

The plants prefer humus-rich, well-drained but not dry soil. Plants from bulbs tend to be slow to establish large colonies. When growing in conditions it likes, winter aconite reproduces easily and spreads readily to form large colonies – almost to the point of being invasive.