I took liberties with your mockup. The two changes I made were to align a groove with the step in the body panel, and drop the lip to align with the reverse step lower down. The curve at the front and rear would move up and out, for tire clearance.
Here's that area handled in a more aero-friendly way:
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FWIW, you saved as a PNG which is good because it carries an Alpha channel. But in the blow-up you can see JPEG artifacts around the curves on the wheelwell. So the file spent part of its life as a .jpg. When you are preparing your collateral materials, that will be important for reproduction.
I don't have Sven7's freehand line (out of practice), but here's what I do with Photoshop:
Starting in upper left, I did a crude Photochop as in upper right. That was printed out and drawn on freehand (not shown). I overlaid that and did a new freehand sketch on a lightbox (a window works in the daytime). This step is weakest, the drawing is merely serviceable.
That was scanned back in. The scanned file got a 2nd pass through Photoshop for tinting and a caption.
Hope This Helps
Edit: 'printed that out, again,' became 'overlaid that'