Turn off the furnace and turn off the electricity to the furnace.
Remove the filter from the furnace. Check your user manual to get specific instructions on how to remove the permanent filter from your furnace. Most furnaces allow users to slide the filter out of the machine after unscrewing a latch.
Take the filter outside and spray it down with a hose if it is a nice day. If not, run water over the filter in your bathtub or basement basin.
Scrub the filter with a soft brush to remove the debris and dust. Continue to run water through it to help remove the dust and debris.
Let the air filter air dry. Insert the air filter back in the furnace. Turn the electricity back on and turn on the furnace.