I cannot locate the filter in my system. I need very specific instructions, as I asked this same question several weeks ago of hvac1000, and still have had no luck. Do you have pictures to assist me? My husband always took care of this task; he passed away 2 years ago and I wish I had paid more attention! Thank you for your help. I know it's pretty simple, but I'm a novice!
Thank you!

It is just about impossible to give specific instructions because sorry to say the air filter could be located in many different places. Some are in return air grills. Some are located in the duct work going to the HVAC unit. Some are located in the unit itself.

Make it easy on yourself. If you husband passed away 2 years ago it is probably about time you have your system professionally serviced. When they come to do the service have them locate the filter for you. This way you will know your equipment is operating correctly and you will locate the filter.