It depends on the appliance. Inside the fresh food compartment on a refrigerator, usually high on the left or right wall. On the edge of the door on a dishwasher. On the frame inside the door on a wall oven or a range. Behind a filter on a hood. Inside the door frame on a microwave, a washer or a dryer. Behind the grille at the bottom on an undercounter cabinet, and inside the door frame on a wine cellar.
Take a photograph rather than writing it down. The full number includes the letters after the digits — those decide which parts fit, and they are the part people misread most often.