I drive a 1969 Citroën Dyane. Because I do off-roads with this car I added an original spec skid plate, the black plate in the middle of the car.

After I mounted the plate I noticed that the airflow is compromised, the car has an air cooled flat twin engine, so fresh air is important, therefore I want to create two extra air ducts.

Step 1: Rough Design

I traced the holes of the skid plate with a marker before i started to disassemble the front

Step 2: Refine the Lines

After i got the front on the workbench, I started to refine the lines. I made two squares and messured both sides so it is kind of symatrical.
In he last image you see that I added the inner lines, these are the cut lines.

Step 3: Cut One Hole

Tools

Angle grinder
Metal cutting disc, I used an 1mm disc for a clean cut
Safty goggles
Ear protection
Sanding paper 40 grit, or a file

Start cutting in the center of the line. Once the disc is trough the metal start moving the disc towards the left/right.
You have to look where the cut is and try to stop on the inner lines.

It is important that you only cut one hole. We are going to strengthening the gap before we continue.

After the cut, deburr the edge with sanding paper or a file.

Step 4: Strengthen the Gap

I don't know if there is an English word for this technique, but we are going to get some strength back in the metal. We do this by making an angle.

Tools

Couple of body hammers
Couple of Dollys
A thin chisel, for the hard to reach places

Take a dolly and hold it firm in one hand and and hold it on the back of the plate. keep it steady and against the plate. Start hammering from the middle and move to the corners. Continue on the straight parts, and wait for the corners.

For the corners take a hammer with a very rounded top to get the perfect radius. It is important to take your time and don't rush it.

Step 5: Repeat on the Other Side

Do the same as step 4 and step 5 on the other side.

Step 6: Clean, Add Body Filler and Primer

To remove the little dents from hammering I added a bit of alloy body filler after i cleaned and degreased the plate.

After the body filler is set, sand it down with 40 grit / 120 grit / 200 grit sanding paper.
After this I cleaned and degreased the plate again and added an primer.

Step 7: Paint, and Mount

After the primer I painted the piece, and mounted it back on the car.

I drove + 4000km in hot weather and the car doesn`t get as hot as before, so mission completed

Step 8: Extra Info

Before I made the holes in my car, I tested different techniques and workflow on a similar piece.
I would advice it to try one, two maybe tree times before start cutting in your car.

This is my first instructable, tips and help are always welcome.