Hi,
It sounds like you are doing fine. For the best results with flowers that make seeds in the head, you don't need to wait until the stems are brown. If the base of the flower where it is attached to the stem is brown, that is a good indicator the seeds are ripe. Often, I will check one flower to make sure that I am correct if I don't know the ripening habit of the plant well.
Expert seed collectors know when seed will ripen and what it should look like. They also know how to examine the seed to separate the good viable seed from the bad. They also know what plants are off limits to seed collection. So try to make sure you don't collect anything listed as threatened or endangered in your area.
If you have any questions about specific flowers don't hesitate to ask : )
Remy