Separate garlic heads into cloves and peel garlic cloves.
Slice garlic cloves into thin, uniform slices. Uniform slices will dry at the same rate and help to avoid burning some slices while you wait for others to be ready.
Lay garlic cloves on your dehydrator trays with room between them for air to flow. Do not overlap.
Set the dehydrator on a low temperature (95 degrees) and set in a place where there is good air circulation (a porch with open windows works great)
Check the garlic after 10-12 hours. When it is fully dried, the slices should break in half with a crunch instead of bending. It make take up to 15 hours to reach this point.
Once dry, use a spice grinder, coffee grinder (used for spices) or mortar and pestle to grind the garlic in batches.
Sift the ground garlic through a fine meshed strainer to keep larger chunks out of your garlic powder. This will result in a fine powder. Larger pieces may have a higher moisture content and cause the powder to clump up; set these aside and use them sooner in cooking.
Place garlic powder into a spice jar or other glass container that seals out air and moisture (or a ziploc bag).