Finding and Cleaning a "permanent type" Air Filter for a Central A/C System
Don't - Run your A/C while the filter is not installed. This can damage it.
Do - turn off power to the A/C system at the circuit breaker panel before working on it. Some parts use very high voltage.
Do - observe where electrical wires are, and be cautious to not move or contact them when possible.
My A/C system is fairly old, so I spent a lot of time systematically inspecting it and finding screws to unscrew in order to remove panels to take a look inside. I eventually found the correct panel, and finding the filter was no easy task. It was horizontal and not easy to see, but once I identified it (with the assistance of a flash light), I was able to remove it.