With the tray removed, take out the old filter.

Note the intended air flow through the filter. The air flow in the heater blower is from top to bottom (down). The tray should have an indicating mark.

Insert the new filter into the tray. The rigid sides of the filter (with printing) go to the left and right side of the tray.

When inserting the new filter, note the retaining tabs inside the tray that the last vanes of the filter fit into, on both ends. This ensures that the filter has a near-airtight seal in the tray.