You want to make some attempt to weigh your dry fiber before you start, because it determines the optimal amount of dye you'll need. This doesn't have to be very exact. You can take on faith the label on yarn balls, you can use a postal scale and add up smaller quantities, you can even use a good bathroom scale and estimate the difference between you weight and that with the yarn. The reason is that you want to aim for about 1-3 grams of dye per hundred grams of dry fiber. You can use less if you want a more pastel look or a subtle overdye, more if you want intense colors, a mix of both for more design interest.