A decoction is a method of extraction of herbal or plant material by dipping, mashing, soaking, steeping or boiling in water leaves, flowers, stems, roots, bark and rhizomes. Making tea would be considered a decoction.
Recipe: Take 100 grams of herb, add 1600 ml of water. Boil down to 400ml (this will take about 2 hours) and take about 100 ml 1-2x/day.
Helpful Links and References:
You tube video on how to do it (OK so the H in herb is silent and there is an “an” instead of an “a” before herb but the video was funny and effective): https://www.youtube.com/watch?v=G35965yQqA4