Taken from s2ki.com forums.
(Yes, there are pictures of a Honda, I am sorry in advance.
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*disclaimer* I take no responsibility if you mess up!
I wanted a vented hood for three reasons;
1. To let heat (from header, engine, & radiator) escape from engine bay
2. To get cooler air into intake
3. To use the low pressure area above the hood to help extract air from under the front of the car thereby increasing velocity under the splitter and floor of the nose which should increase downforce.
Why not buy a carbon fiber vented hood? The affordable knock-off carbon hoods are heavier and less stiff than the stock aluminum hood. I believe the stock hood weighs 14 pounds. The high quality "authentic" hoods (cwest, mugen etc...) cost a bundle and still need to be painted unless you are going for the carbon look. Some if not all composite hoods require hood pins for high speed use. Oh yeah, and I'm cheap. I planned on venting both sides but ran out of time and decided I like the look of just one side. Still debating.
Here's my solution:
How to build your own:
Please use appropriate safety equipment and don't screw up.
Yes, you are about to cut holes in a perfectly good hood so be prepared.
Remove the hood and the hood liner and support it upside down on some padded saw horses.
Make templates by laying paper over each support hole you wish to cut and trace the edge (use the pencil rubbing technique). Use the templates to cut out some perforated aluminum (clean and paint the both sides of the aluminum). http://www.mcmaster.com/
Use a 3/8" spacer against the hood support edges to draw the hole shape. I left a 3/8" lip for mounting purposes.
Using a UniBit, drill a hole in each corner.
Using an air saw, cut out the shape from the under side - leave a small gap to finish off with sanding.
Use a sanding drum on a die grinder to smooth to shape. I tried using a sandpaper wheel on an angle grinder but it gets too hot and the paints starts to peel back.
Once the shape is close, flip the hood over and apply masking tape on the top side all around the new openings.
Use wet dry sandpaper to smooth the cut and feather the paint. The masking tape is there so if you slip with the sandpaper, you won't scratch up your hood.
Lay 3M double stick tape used for moldings/body work on your 3/8" lip and trim with a razor to fit just inside the cut out (find it at automotive body shops).
Use a fine paint brush and carefully paint the exposed edge with factory touch up paint.
Install perforated aluminum and your done. I used some small sheet metal clips to hold the perforated aluminum in place just in case the double stick tape failed due to engine heat but I ran a full track day without them and everything was fine.