The seeds need cold stratification to be viable, so planting in fall can yield plants in spring. You can also direct sow in early spring. In either case, sow the seeds by pressing lightly into soft damp soil. The seeds are tiny so watering them directly may wash them away; instead try misting them lightly with a sprayer. They need a fair bit of sunlight to germinate in spring, so consider the seasonal placement of the sun in your garden when planting.
You can also grow rose campion seeds indoors, just remember to cold stratify before you plant them. In this case, just put the seeds in the fridge for about 1 month.