cfair

Not necessarily the best process, but one I've seen work well:
Big CA letters are 1/4” wide, CALIFORNIA are 1/8” wide, both embossed from rear.

Aircraft stripper removes old paint. Mask off any registration stickers so as not to lose them.

After chemical stripping, lightly sand metal surface for good paint adhesion & uniform surface. Hammer & straighten letters and black field as necessary for straightness & embossing. Use thin coat of bondo if really going for it. Spray with Prep All. With stickers masked off, spray the plate with 2 light coats of Krylon School Bus yellow. Let dry thoroughly between coats.

Use 1/4” blue tape to mask off straight lines. Burnish tape well to keep black from bleeding under it. Consider using masking fluid for curves. Mask off slightly more yellow than necessary, then use black sharpies to clean up jagged lines after shooting black layers & removing tape.

After straight lines & curves are masked, shoot 2 light coats of Krylon Gloss Black. Let it dry thoroughly between coats. After 2nd coat is dry, remove masking tape & fluid.

Lines between colors will probably be jagged due to paint layering over masking tape / fluid.

Clean up the lines with black sharpies progressively with large to small sharpies: Sharpie King size for big gaps/mistakes, use mid-size for smaller, Sharpie fine for final lettering & curves.

After lettering is cleaned up, shoot the whole plate (less masked off stickers) in satin or gloss clear coat to preserve the work from gasoline and other solvents.