Updated: Oct 10, 2019
This is the method I've used for growing annuals from seed and which has worked well for me. There's no one correct method of doing this and this method can be adjusted as people see fit.
I don't use seed flats as they take up a lot af space and are too large when only growing plants for personal use. Unless you want to grow a large number of plants of one sort, then a seed flat is the way to go. On the other hand, if the intention is to grow a small number of several varieties it's better to use smaller containers. Here in Iceland and type of yogurt comes in the perfect containers for this purpose with a very handy plastic lid. I don't know what is available in other countries, but hopefully something similar.
1. Container washed and labled
The container of choice is washed and a few holes punched in the bottom. It's important to label the container (not the lid!), I use white tape and write on it with a permanent marker.
2. Soil mixed
Use a good quality seed starting substrate. Sometimes it's better to mix in some kind of grit to make it lighter. Here in Iceland we're blessed with an ample supply of volcanic pumice which is very well suited. The mixing ratio depends on the substrates used and what you are planning to grow. Some plants need good drainage and then you need to mix in more grit. Using water crystals helps to keep the soil from drying out. I mix the substrates in a large bin and start by putting in a measuring spoon of the crystals and 1/2 l of water. I let the water soak in and then mix the the substrates. You can also mix the water and crystals in a pitcher and mix it into the soil when the water has been absorbed. Either way works fine.
3. Filling the containers
I fill the containers to the rim as the soil will settle. I there are large clumps it's best to remove them or try to break them up. The the soil is lightly compacted (here the plastic lid comes in handy), but it's important to not compact it too much.
4. Sowing the seed
The seed is drizzled over the surface as evenly as possible. Folding a small piece of paper in two and using it to spread the seed makes it easier to spread the seed evenly.
If the seed is very tiny, it is not covered, just pressed into the soil, here the lid comes in handy again. Larger seeds are covered with a thin layer of soil. It's better to use less than more when it comes to covering the seed.
5. Watering the soil
It's best to let the water soak in from the bottom. The lid is very well suited or this purpose. Let it stand until all the water has been absorbed. This takes a few minutes. Discard any water that is not absorbed. You want the soil moist, but not wet.
6. Waiting for the seed to sprout
The container is closed with the lid and placed in a light and warm spot, but not in direct sunlight. A window facing North is best. Then you need to keep watch to see when the seed starts to sprout. It usually takes 1-2 weeks for annuals to sprout. Sometimes it takes just a few days, so it's important to check the containers every day.
7. The seed sprouts, then what?
When the seed sprouts it's important to move the seed container to a cool, well lit place. It's best to keep the lid on while the plants are small, so they don't dry out. When the first true leaves appear it's safe to remove the lid. Then it can be used as a watering dish.
To keep the seedlings from becoming leggy it's important to keep the temperature down and provide ample light. It's fairly easy to set up a seed starting space. A garage or cool storage area is ideal. If there is a window, it's best to put up a shelf in front of the window. The light from the window is not enough though, you need an extra light source. Flurescent lamps are very effective. Lamps with two bulbs are best, using one "Cool White" and one "Warm White" works just as well as buying the more expensive grow bulbs.
If there's no garage you can use an inside window if the room isn't heated too much and you can fit a flurescent lamp there. My space is in the wash room, not ideal, but it works. Windows facing north or west are best as young seedlings are sensitive to direct sunlight.
My seed starting space. I keep the containers there until the seedlings are ready for transplanting.
8. skref: Transplanting
Now the plants will need more space. When the seedlings have 1-2 pairs of true leaves it's time to transplant then into individual pots. 7 cm pots are enough for most varieties, but some might need 10 cm pots. If the pots are available in different depths it's better to choose the deeper type to give the roots more space.
9. Growing on
If you have a greenhouse you should be able to move the plants there in April here (probably sooner in warmer climates) A cheap heat blower can keep the temperature above freezing in cold spells.
If you don't have a greenhouse a warm frame can be an effective solution. I have flurescent lamps in the frame and heat it with a small heat blower. I have a thermometer that shows min and max temperatures and let the heat blower run for about two days before I move the plants outside, to make sure the temperature is set right. I set the heat blower so the temperature is 5 - 20°C. I start using the frame as soon as the plants are ready to be transplanted, usually in February. When it starts to warm up in spring the plants can be hardened off by opening the frame when the weather is nice.
The plants tucked in nice and warm despite snow and frost outside.
Stock flowers in late May 2015, ready to be planted outside.
10. Planting outside
Here in Iceland the time to plant annuals is in early June. It can be tempting to plant them sooner, but late frosts can be expected throughout May and even the first week in June. In warmer climates plants can be planted out after the last frost date.