The air filter on a Toro 20331 22" High Wheel Recycler mower (as well as some other Recycler models) is on the left side of the engine, as you're looking at the mower from the front. It's hidden by a black plastic cover, which can be removed with either a flathead screwdriver or a socket wrench with the correct size socket.
The screw is located toward the top of the air filter cover. (It's the hex head screw, not to be confused with the round screw located on the top side of the mower.) Remove the screw and then pull the cover out and down to reveal the air filter, which is just a rectangle consisting of pleated paper. Take this filter out and then wipe down the filter casing to get rid of any dust or dirt on it. Then install the new air filter (part #491588s) in the same orientation as the old one, with the pleats facing outward. With the new filter in place, push the cover back up and put the screw back in. That's all there is to it.