Ooh! So glad you asked! *grin*
Herbs are funny things, in that some like wet soil, some dry... some like airy spots, others like shade. Some bright mind created the herb (or flower or whatever) spiral to address that! You start out by making a stone spiral that is no bigger from edge to center than the length of your arm (that allows you to reach into the center to weed or harvest). I have used field stones and bricks on two different occasions, and I like the bricks a lot because I can space them with the occasional hole which both allows water to come out if it's raining hard and you can plant pretties in it like snowdrops. You want the center to be higher (in other words, as the edge of the spiral works inward, make it higher and higher) so that you can put bigger herbs there.
You cover the grass under it with paper bags that are cut open, then throw down some good quality straw (not hay, as you don't want the seeds!). Soak it until it's *really* wet. Add dirt, then more straw, then more dirt. I used well composted dirt and aged manure mixed well together. When it's full, you let it sit for a day or two to allow it to settle, add more dirt if necessary, then plant your herbs (or flowers or whatever). I usually put things with big roots up at the top, and/or herbs that like drier soil. Things that like very wet soil go down at the bottom along with shade loving plants. Oregano does well at the middle level on the side that gets good but not full sun. Dill does great up top, or down on the sides, depending on if it's mammoth or not. Thyme can be up near the top as it tolerates the dry better. Rosemary goes near the bottom as it loves water. Basil can go near the top if you aren't growing too much of it (more than a couple of basil plants will overrun the spiral).
There's a GREAT video on youtube about it: http://www.youtube.com/watch?v=YcODj6vcles
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