Spread your coffee grounds in a thin layer on a baking sheet.
Put your oven on a low heat setting (around 200° F) and let the coffee "bake" for about one hour. Once your coffee is completely dry and is a bit darker, it is done.
Take the baking sheet out of the oven and let the coffee cool off. Once it does, grind it up using a coffee grinder into a fine espresso powder.
Store the powder in an airtight container. This is to ensure that it keeps all its flavors and aromas (and that it does not go stale).