The important point is that you need to not only get the dye onto your fiber, but you need it to chemically bind, to actually become part of the fiber. This works best if you use the twin secrets of acid dyeing: acidity and heat. It doesn't take much acidity to do the trick, the general rule is a glug of vinegar per potful. That's right, a glug - a scientific measurement of what happens when you just tip the bottle. A small glug will still work, perhaps not so well if you're using very alkaline well water. A large glug will work well if perhaps a bit too quickly, and pure vinegar won't work any better. A small point to remember here is that acidity doesn't harm protein fibers, in fact slight acidity is good for them. Remember how grandmas used to rinse their hair with dilute vinegar or lemon before commercial conditioners came about?