There's a reason that dude drives a Hyndai
Anyway - if you are serious about it...the idea is that the foam is the "mold"...
then the fiber glass (a woven fabric material, usually available at any marine type store, as this is what boats are made of and repaired with, or probably your local hardware store has some type of fiberglass mat/roll available) is drenched in resin, (a honey like material) and laid over the foam (don't forget to use foil as a protective barrier to keep the resin from disintegrating your foam 'mold'

) .....
then the resin cures (dries) to form a solid sandable skin over your foam mold....
you sand the fiberglass and use "putty"

to finalize and smooth the shape...
Oh yeah...don't forget to paint and then attach the "kit" to the car...something our friend with the Hyndai forgot to add to his "how-to"
Companies that sell kits have already done all this hard work by creating a urethane or fiberglass skin on a mold...then they sell it to you as a body kit. I personally would recommend you take this route, instead of re-inventing the wheel like Mr. Hyndai owner with the Home Depot Kit