Assuming you have collected seed from a plant right now whilst the seed is fresh. Either scatter over the open ground, to imitate the way the seed is scattered in nature or sprinkle over the top on a pan of potting compost and cover very lightly with grit.

As MG says...............But a related Q from me.
The poppyseed I use in the kitchen bought in the Supermarket, is it likely to be viable ? Poppy seed can lay dormant for years

You could try but I would be doubtful you would get any plants... the seed will have been irradiated to prevent it germinating.

MG

It would also be helpful to find out what kind of poppy, and where in the world you are, Maggie. In hot summer, mild winter climates like mine, we sow all poppies after the soil cools down in late fall--about November, here--and don't expect too much from the Oriental poppies. In areas with cold winters, California poppies do much better sown in early spring. Most of the UK is normally sort of in between, though the last two winters could easily count as "cold"!!