Device is a noun. It is a tool of some sort.
You might use many devices to carry out the plan you devised. Remember them this way: a tool is a concrete thing. You can pick it up and hold it in your hand. Device has a “c” for “concrete”. An idea is abstract. If you tried to pick it up, it would slip through your fingers. Devise has an “s” in it, for “slip through your fingers”.