The flavouring is usually in a dry or solid form, as opposed to liquid. The most common seasoning base is just salt and pepper, but paprika, dried or chopped fresh herbs, are also often added.
If you are following a recipe, most recipes will tell you what to season the flour with. Some recipes for seasoned flour, such as those for coating fried chicken with, will be quite elaborate.
Most people reach for white flour as the base, but whole wheat flour can brown up more nicely, and also have more depth of taste. Flours other than wheat-based flours, such as nut flours, can also be used.
You can buy seasoned flour mixes ready to use both for consumer and commercial use. You can also use these already-prepared seasoned flours for thickening gravies, sauces and soups.