A decoction is usually made from roots, bark, seed and dried berries rather than fresh or dried leaves. It's still similar to making tea, only this time you add your herb to cold water, bring to the boil and leave to simmer for a while.
Herb teas or herbal infusions are commonly used immediately. Decoctions can be kept in the fridge for a few days and used when needed.
To make a decoction, add one tablespoon of dried herb or two tablespoons of fresh herbs to 750 ml cold water. Bring to the boil, turn the heat down and simmer for 20-30 minutes. Strain into a clean container and cool. Use as needed.