To find it:
1. Pop the hood: Open your F-150's hood.
2. Look for a large rectangular box: This box will likely have a large hose connected to it, leading to the throttle body. This is the air cleaner housing.
3. Locate the access panel: The air cleaner housing will have a panel, usually held on by clamps or screws, that you can remove to access the air filter. It might have a clip or latches.
If you're still having trouble finding it, a quick online image search for "1990 Ford F-150 air filter location" along with your specific engine size (e.g., "1990 Ford F-150 5.0L air filter location") will likely provide pictures showing the exact location. You can also consult your owner's manual.