I'm new to gardening, and although I've studied a lot apparantly a picture is worth a thousand words. Lots of sites show you pictures of herbs in the prime of health...and even in bloom, but FEW show pictures of what they look like when they've produced a seed.

I have yarrow and it's currently in bloom. When will it make seeds, how will I know when they are there, and how do I harvest them?

Add basil to that list as well...how do I get the seeds from that too?

Basil makes a tiny seed pod just beneath the flower. As the flower fades, the seed pod grows. It never gets very large, the seeds are itty bitty.
When the pods are recognizable, pull them off and set them in a dry place. When they are totally dry, they will fall apart. They seeds are small and prolific.
Yarrow is a little harder. Each segment of the large flower cluster will produce seed. Yarrow, though, is one of those plants that can produce seed after it is cut. The best way to collect is to cut a few flowers and hang upside down to dry. As it dries, you can examine the base of the flowers for the little seed pods.
If you would rather wait and collect from the garden flowers, watch to see when the flowers fade and brown. You can simply crush them over a saucer and separate the little seeds from the chaff. The seeds are grey and smaped like tiny cat's eyes. They generally have a stripe through the middle.