Separate the garlic heads into cloves and peel them.
Slice garlic cloves into thin, uniform slices. Uniform slices will dry at the same rate, helping you avoid burning some 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 it in a place where there is good air circulation (a porch with open windows works great)
Check the garlic after 10-12 hours. The slices should break in half with a crunch, not bend, when fully dried. It may take up to 15 hours to reach this point.
Once dry, use a spice grinder, a coffee grinder (for spices), a mortar, and a pestle to grind the garlic in batches.
Sift the ground garlic through a fine-mesh strainer to remove larger chunks and keep your garlic powder fine. This will result in a fine powder. Larger pieces may have a higher moisture content, which can cause the powder to clump; 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).