Poppies can grow pretty much anywhere, even indoors. However, they do prefer to get a lot of sun in their lives, so outdoors is always best. If you're planting in a garden, all you need to do is cast the seeds out and let them fall where they may. You can cover them with a small amount of soil, only about a quarter inch, to give them a bit of extra protection from the elements. Poppies don't transplant well, so if you start them off in pots (indoors or out), then make sure to handle them very carefully when transplanting. Peat pots work well for starting out too, as you can plant the pot as well and it will biodegrade. This way you don't run the risk of hurting the poppy's sensitive root system.
There are two main outdoor planting times for poppies -- early spring and late fall. That's right, fall. If you are in a more temperate area, then they should survive the winter in a dormant state, even if covered with some snow.
The optimum germination temperature is around 60f (15c), although they will still germinate if the temperature is higher or lower. Don't forget to keep the soil moist, but not over-watered.
Once your poppies begin to sprout, use a spray bottle and give the seedlings a good few sprays to moisten the plant and wet the soil. The roots are extremely sensitive at this stage, and a big waterfall from a watering-can could kill your plant on the spot. Try to water in the morning or the afternoon. Night-time watering can encourage fungus growth. Also, try to give your poppies as much sun as your situation will allow. If you pay close attention to your plants, and maybe experiment here and there with some fertilizers, you'll most likely have an incredible looking poppy garden within a few months.
Ideally, you should try to give each plant as much room as you can. Roots should have at least a foot, preferably more, of soil depth. Plants should also be spaced a minimum of a foot apart for best results, and to prevent their root systems from intertwining. You don't want them to have to compete too hard for nutrients, so planting them too close will likely cause underdeveloped plants. But if you're working in a small area, then so be it. You just have to work with what you have.
Situation:"I have a small problem, I have 4 pots 3 plants in each. I know you should stop watering as soon as the petals drop but some of the plants that have pods still have buds that haven't flowered yet. How do I get around this? Can I water them VERY lightly so I still give the plant enough water to sustain life and flower those other buds but yet little enough so I won't dilute the final product?" -- APrime
Advice: "The later maturing pods will usually not produce very well, so it's better to stop the watering and milk the healthy ones. The later ones will rarely make it anyway, so milk the healthy ones. Stop watering when the petals fall off so that you get good production out of the first ones. BTW, the plant will try to produce until it dies." -- roadrunr
Rock advises: After I have gotten what I want from the first pods and I can see that the plant is producing more, but the leaves are turning yellow from lack of water, I water lightly. The reason is because the plant and pods will die from lack of water. When watering in the future, every 2 weeks water deeply so when it comes time for harvest, the soil will be moist where the roots drink from. A poppy plant only grows it's roots down a few inches, then the roots grow horizontally. This is also why spacing is crucial when planting. Always water 10" out from the stem because the roots grow away from the stem, not down. Keep the soil moist in a wide area around the plants.
For Papaver Somniferum varieties, it is best to use HID lamps (HPS or MH). A 1000W can shed ample light to grow a 12ft by 12ft area. HIDs get hot so you may need to cool the room or find a plant that grows in a warm climate.
Photoperiod for germination and the vegitative "cabbage stage" should be 12/12 (12 hours on; 12 hours off).
To force flowering use an 18/6 or 24/0 photoperiod.