These (linked) videos will provide inspiration on HOW TO make professional/factory looking aero body panels etc for your car and get you TF AWAY from freaking Chloroplast and flat plates etc beaten to dentey death thinking..!!
I chose that video for its 'adverting' pic. The whole channel with way more examples:
https://www.youtube.com/c/NH%E1%BA%BETTV/videos
Having watched some of those:
From my practical (glider and aircraft building and panel beating) experience, I can add:
NEVER sand anything by hand! EVER!
EVER!!!!!!!
ALWAYS use a sanding block that already has the basic shape of the surface you are trying create and is as long as, or longer, than the sandpaper being used. (the longer the better)
(A decent wood plane helps create these sanding blocks and nowadays we have 3D printers)
Use a cross pattern.
eg: if sanding a cars roof; sand front to back, then sand left to right, then front to back again, etc-etc, using a slightly concave sanding block. NOT!!! by hand and NOT!!! one of those rubber block things they have at hardware stores.
Spraying on a very thin layer of cheap paint between sandings will highlight dents in that the paint is sanded away on high points and remains in dents, indicating where more body putty, spray filler, plaster of paris, clay or whatever is required.
If there is a harder surface underneath (normally metal) STOP sanding as soon as (to just before) it becomes exposed: It will NOT sand away as fast as the surrounding areas and will remain a high spot.
If you cant add more filler to that area:
Sometimes a small hammer can be used to dent such a 'high spot' inward, sometimes a dremel. Etc. Then apply more body putty or whatever you're using to that spot.
When you're down to the final sanding/s before applying the top coat; add soap to the water you are using to sand with.
This gives you a shiny, reflective surface so you can more easily see any imperfections in the reflections off of the (wiped clean with a damp to wet cloth) said surface.
The final sanding/s (of the applied undercoat) need to be done with water paper with a grit of no coarser then 400 grit: any coarser and the sanding grooves will be visible through the top coat you are about to apply.
It's best to have the top coat applied by a professional with a paint booth.
This can be DIYd but any runs at this point are something of a disaster.
The top coat (once FULLY dry) is the one and ONLY time you can give the surface a quick sanding (flatting) with very fine 1000 to 1200 grit Water-Paper by hand before a HAND polishing with the polishes normally used on brass or copper ornaments. (a very fine abrasive polish Called Brasso locally. Much rubbing, like sanding, reqd)
This 'flatting' is to get rid of any dust and fluff etc that got onto the wet paint, forming tiny high spots.
The plug has to look exactly like, or have the finish of the finished product, as any imperfections will transfer over to the outer female mold you make of the surface.
You then use the female mold as a mold to make the final panel.
All this Body Putty, Spray Filler,WaterPaper, mold release agent/layer, glass micro beads etc (and advice) are as far away as your local hardware store, as is the required (large enough) air compressor and spray guns etc.
ie:
These are not high tech things available only from God and magically manifested only at the factories of fully fledged car manufacturers!
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