In her book The Herbal Kitchen, Kami McBride gives quite a few recipes for herbal sprinkles and using powdered herbs in honey, butters and other culinary uses, which you can find how to's for here.  As I have used these recipes and countless others calling for powdered herbs I have found the need to turn my garden herbs into powder form.  

Step 1: Dry Your Herbs

I have already given a tutorial on drying herbs here.  I also love to make powders from vegetables but these must be dried a certain way depending on the vegetable you are turning into a powder.  You can find detailed instructions per vegetable in the preserving the harvest section.  As a simple review herbs can be dried in a dehydrator, on a screen, basket or hanging in bunches.

Step 2: Crushing into a Powder

This can be achieved by several methods and can also depend on the type of herb you are crushing.  A quick note here, the more an herb is handled, crushed and powdered the less potency it will have.  Because of this I always store my herbs in as whole of form as possible and then powder them right before using.  My preferred method of powdering is in my Blendtec Twister Jar for large amounts and a simple coffee grinder for smaller amounts.  You could try a regular blender but they just don't have enough power to turn anything into a powder.  Another option is with mortar and pestle, many people swear by these for crushing their herbs but personally I found it a pain and the results weren't any better than my coffee grinder.  After powdering the herbs shake through a strainer to remove larger particles that didn't get powdered.  

Step 3: Storage

If I am making an herbal sprinkle I just use old spice jar containers or some really cool jars I pick up when I am out and about.  You can also purchase empty spice containers and I always suggest glass which will keep your herb potent longer and keeps the flavor bold.  Try to store in a cool, dark place for a longer shelf life.  Label with the date you powdered the herb and what it is.  Once your powder loses it color and aroma it is no longer medicinal and time to discard into the compost pile.

Powdering Roots

When making an herbal powder from a root, make sure to wash thoroughly to remove all dirt and debris and dry using the dehydrator.  Try whichever powdering method works best, but from personal experience if you want a real find root powder you need to purchase your herbs in powdered form from a good quality herb store like Mountain Rose Herbs or Bulk Herb Store.  The commercial grinders they use to powder the herb is the only way to get that smooth, fine powdery texture out of a root.