A decoction is just a fancy word for making tea with roots, bark and hard plant material. To make a decoction you need to not just steep it in hot water but you need to put the herb in water in a pot and then bring it to a boil and steep it for a bit of time- usually anywhere from 30 minutes to many hours.
Some herbs you would decoct: burdock, cramp bark, licorice, dandelion root.
The dosage of how much root to use really differs depending on the herb. A standard decoction would be about an ounce of a root to a quart of water. If you were to measure this out, its usually about a 1/4 cup
of root or about 3 heaping tablespoons. Generally many herbs are decocted for about 20-40 minutes.
When you read the herbal section, I will often suggest decocting 1-2 tablespoons of a root per day. To do this, measure out your herb and pour it in a pan. Take a pint of water (16 ounces) and add in 1-2 tablespoons to your pot. After bringing it to a boil and simmering it for 30 minutes the herbal material will absorb some of the water and some will evaporate off. You will end up with about half the volume- or about a cup of tea to drink.
Some herbs do better with really long decoctions such as reishi- which can be decocted for 24 hours to really extract its value. Others such as astragalus and codonopsis are often added to broths that are also simmered for many hours with good effect.