It's been asked a lot, and so I've decided to post the way I do it. This isn't really the best one to show since I didn't really score the tube very well, nor did I taper it and the detail isn't that good, but it's good enough to show you how to do it.
1. You'll need a length of styrene rod, or tube. Mine is 3/16. You'll also need a saw with teeth that aren't too close together, but mine are a little too far apart for this. If your saw is like mine you can do it twice to get the right spacing between the folds of the filter, since that's what you're going to do with this. The mitre box is just to help me, it's not totally necessary.
2. Lay the rod in the mitre box and press the blade against the rod. Then drag the rod under the teeth, the direction the arrows are pointing. This will score parallel lines in the rod. It just occured to me that I could have put the saw in one of the 45 degree angle mitres and it would have made the scorings closer together... oh well. Anyway, do this several times to deepen the lines, and then several more times all the way around the tube until the entire thing has been scored to look like an airfilter.
3. Saw out a portion to use as the airfilter. I'm using the second peice I cut off, the first is too small for my taste.
4. Punch some holes in styrene card, but keep the holes. Glue these to either end of the filter, and sand them to close to the same diameter. I've found it's easier to make a round thing smaller and round than a square thing the same size and round. Just the way I do it.
Glue the filter to a bit of styrene rod (this was bent from sprue) for the piping; paint and detail as you see fit. The connector is made from tape, painted, with metal foil on it. And there's the finished project. Simple, no?