Depends on what you want to do after the decals go on. If you want to use some sort of wash (to bring out recessed detail, panel lines, etc.) the order would be slightly different. Gloss coat, place decals, gloss coat again, apply wash, (it will spread easier with a gloss coat) then use the dull coat.
As to what products to use for your gloss coat and dull coat, that's a whole 'nother discussion.