I have collected seeds from miniature daffodils. How and when should I plant them?

You'll need a free draining compost, 3/4 fill a shallow pot and firm down,
Sow the seed and cover with a thin later of compost, just enough to cover seed.
Cover that with a fine layer of grit, label and stand in a shaded area.
The seedlings look like grass after about a year but leave in thiere pot another year to form 'bulbs'.
After the second year is over, repot into a gritty compost.
Daff seeds can take 4-5 years to reach flowering size some other bulbs take even longer to flower!

Good luck and hope your patient!