This video explains basic fiber extraction from dogbane. There are other natural fiber producers, the top three I like to use and are most common in my area are: Stinging Nettle (Urtica d.), Milkweed (Asclepia s.), and Dogbane (Apocynum cannabinum).
Dogbane, also known as Native or "Indian" Hemp, is one of the strongest natural plant fibers to be found. In its younger stages it looks very similar to milkweed, which also produces pretty good fibers. It is best to harvest the brown stalks of dogbane in mid-late Fall and early Winter. It grows in sunny moist areas: ditches uphill from lakes and ponds, and often grows where you will find its look-alike, milkweed.
Whereas milkweed and its parts are edible in is varying growth stages, dogbane is not! In fact, it is know to be toxic! So be careful when harvesting milkweed shoots to eat, because dogbane shoots are almost identical.
Primitive technologist, Dino Labiste, has more videos on dogbane at PrimitiveWays.com.