Harvested seeds

I tend to keep them in a paper bag (never throw one away) overwinter and plant in a seed tray at the appropriate time in spring. I keep the seed tray in a clear plastic bag (again - never throw one away) until the seeds sprout.

An exception are pansy seeds which can be planted anytime. I had a fabulous display of pansies and have collected the seeds and planted them almost straight away. They are now in small individual pots waiting for my petunias to finish and then will be put in the front garden. Poppies tend to seed themselves but no harm in planting some seeds.

Good luck. It is a lovely hobby and very rewarding.

As Liz46 says. It also is worth looking up germination of each type, as some seeds rely on the natural cold of winter to trigger germination. These type of seed benefit from a short time in the fridge before sowing.
Some seeds such as Love in the Mist and Marigolds will grow naturally outside where the seed has fallen, as will Nasturtiums.
But I like to gather the Nasturtiums and start them off myself indoors. They seem to grow stronger and flower earlier that way.

The varieties mentioned are all hardy enough that you could sow the seeds now. Some of them may not flower next year if you wait until spring.
Why not save half and sow half.

I do bothBlueBelle for example, I’ve scattered some ripe aquilegia, foxglove, wallflower, etc seeds straight from the pods near where I’d like them to germinate, and the remainder I’ve scattered into labeled seed trays with Jack’s Magic compost. These are currently outside. These will germinate and I’ll fetch them into the greenhouse to overwinter.