how to make louvers on 1/25 cars?

Dave - You might want to check around and see if anyone has some of the old (VERY OLD) customizing pieces from the AMT kits of the 1960s and 70s. Included in the Lakes Pipes and hood scoops were a strip ot two of "louvers". If you can get a couple of these,, snad them down a bit and glue them to the hood of your car (or where ever) and coat them with a little white glue to fill any gaps. After painting, don't forget to touch-up the "holes" with some black paint carefully applied. Good luck!
-- John The history of things that didn't happen has never been written. . - - - Henry Kissinger

Hey Dave,
I've got some of those louvers John was talking about in his reply to your question. Send me an email personally with your address if you want to try them out. I've got them packed away but if you want to try that I'll dig them out.
Mike G.

You might try shaving them off of a Locomotive shell.

You need a trimming jig. If you don't want to make one, there is that razor chopper thing that Micro Mark sells that has a length duplicating adjustment. I have used the quarter round stock and it works okay.
You can also use thin card stock or styrene, cut the louver slots to size, and use a small screwdriver to push down the louvers. Use a piece of balsa or basswood as backing when you do this, to allow the backing to give under pressure from the screwdriver (a small one, of course). The backing is then glued on as an overlay with louvers facing outward. If you want inward facing louvers you need to recess out the area behind the louvers.
This overlay panel must be cut to the size of an existing panel on the car. Works fine on race cars, which have lots of removable panels. It is a bit harder on a custom, as you'll need to smooth the overlay down into the existing body.