Onion seed sets are basically onion bulbs that have been picked and stored during the winter months so they can be planted straight into the ground the following spring. They will usually be dug up and dried over the winter so that they're ready to be transplanted straight into the garden the following year. Since they're already a year old, they won't produce full-size bulbs at all without being started inside. So you might as well plant them straight into your garden if you want them to produce fully grown bulbs.
All varieties of onions have their own set of "black seeds". These are used for seasoning sauces and soups. Their seeds are also edible, although it is not recommended that you eat them straight from the plant. You should instead boil them in water to make sure that they retain most of their nutrients and start to break down into smaller pieces. After boiling, drain any bits of water from the seeds, and discard them (along with the seeds) in a bowl.
The "white" seeds are called "red seeds" and are only available in white or sweet varieties. They don't have the black seeds in them that makes them different from their black seed relatives. To prepare these, you simply need to dry them in a dry place for a few days. Then, remove the pin-like cores and grind them to make them into a fine paste.
If you're planting them into a garden already in place, then the only thing you need to remember is that you need to dig them up before they're fully developed and ready to be started indoors. Digging them up in the first year of planting will ensure that they don't get lost and be taken care of before they flower in the second year. Don't try to save the seeds from the last frost as that's when they'll be most likely to sprout. It's best to just take them out when the weather gets cold. When spring comes, you can put them back in the garden.
You may find that your garden has a full complement of onions already. And if it does, you can easily transfer those onion seeds into your indoor planting set, keeping the old ones to harvest in the fall. Just be sure to keep your onion sets in an area where they won't be disturbed. Also try to spread out the onion sets on a flat surface. That way, if you plant more onions in a certain spot, there won't be a problem finding a nice, even size lawn to plant them in.
To help you get started with your indoor planting, make sure you follow the instructions on the package or printed info leaflet for the proper amount of time to wait before you plant each onion seed. Usually, you have at least a year to go until the next biennial crop. So choose the proper time to move your onion seeds to a dry place so they have time to grow, bloom and create tasty, colorful bulbs for you in the coming year.
If you plant your seeds too early, it will be tough for them to develop. Also, if they sprout too early, you could end up with small, developing shoots that are too small to support a healthy root system. You want your onion seeds to produce big, healthy leaves and blooms that will be attractive and colorful. If you plant them too late in the life cycle, you risk having little or no leaves at all.
There are many reasons you should be taking time to plant your seedlings. Some of those reasons are more about the design of your home garden. A beautiful garden can mean the difference between sitting on your couch watching your favorite TV program and spending a fortune on cable programming. And that is exactly why planting your black onion seeds early is important.