Heil furnace filters come in a variety of sizes to fit your type and model of furnace. They also come in different composites, from standard paper filters, foam reusable filters, and charcoal active filters for getting rid of odors from pets or smoking. You will have to make the decision yourself as to which type of filter you'd like, but they all install in the same places and in the same way.
Remove the lower panel on the front of the furnace. It is held on by gravity clasps hanging on a rim and will just need to be lifted off and placed on the side.
Find the location of the filter. Depending upon the model you have, whether it is an upflow or a horizontal flow, the filter will either be on the side next to the air return or on the bottom.
Pull out the entire filter holding rack. Remove the end cap.
Pull out the filter after the end cap has been removed. Push a new filter inside.
Replace the end cap and slide the filter rack back inside of the furnace. Replace the front cover by sliding the clasps over the rim and allowing gravity to settle it in place.