Disconnect power to the furnace by shutting off power at your home's breaker box. Locate the metal panel on the front side of the furnace. Remove any retaining screws or bolts securing the panel in place. Grasp each side of the metal panel on either side of the furnace. Lift the panel off of the furnace and set it aside.
Grasp the oil filter revealed by the furnace's panel cover. Slide the oil filter out of its slot and place it on a clean surface. Avoid getting any dirt or debris in the filter.
Locate the fan assembly mounted to the right or left side of the furnace. Loosen the nuts of Phillips-head screws mounting the fan inside the furnace. Slide the fan out of the furnace. Set it aside.