I've seen better than pro looking results coming from a rattle can done by folks with a little bit of practice, and a HOT garage...
Seriously. To DIY, you would need the following supplies.
#1. Rubbing alcohol
#2. Tack cloth, or lint free micro fiber.
#3. 300 grit or finer sanding sponge.
#4. Distilled water.
#5. Duplicolor Adhesion Promoter. (Flexible plastic primer).
#6. Duplicolor color coat for your paint code.
#7. Duplicolor clear coat.
#8. Hot garage.
#9. Trash can, bench, or whatever to rest flares on to cure.
Oh yeah, the flares themselves.
Rub the flares down fully with rubbing alcohol and the tack cloth. You want to remove any lubricants, sealants, and that sort of crap that may be left on the flare in the manufacturing process.
Lightly sand the flares down using a wet sanding sponge. Rinse the flare and let dry.
Spray per can instructions, the adhesion promoter. Put flares in garage and allow to cure at least 24 hours. Repeat to insure complete coverage of the flares.
Spray per can instructions with color coat, allow to dry, and LIGHTLY wet sand between coats. Depending on how light your coats are, 4-8 coats will do the job. You want a smooth blemish free finish. Wet sand last coat lightly, and rinse / allow to dry.
Step #5. Spray per can instructions with clear coat, same as with primer and color coat. 2-4 coats of clear is fine. Allow this stage to cure in the hot garage at least 5 days prior to assembly.
Step #6. Assemble flares onto the truck. DO NOT USE HARSH ABRASIVES OR WAXES FOR AT LEAST A MONTH!
Now you know why pro painters take so long, and cost so much.
DIY isn't hard to do, just time consuming.