Alpha Map/Mask

hi guys,

i've been searching around the internet for a way to make alpha maps (for decals that would be applied on cars), but couldn't find anything conclusive.

Could anyone explain or direct me to a tutorial on how to make them?

my attempts on making alpha maps were not good since selecting the outline of the decals with the wand (in PS) gave it an AA, and on the car the decals outline were transaprent