There are extensive discussions of best practices for application on various online fora -- I don't recall on this forum, but you can certainly find them on the GHQ Forums.
For me, though … I've never been skilled enough to manage the fiddly part of decals at this scale. The danged things just float around too much, then set in the wrong place / at the wrong angle, and then get torn when I try to move 'em.
So I paint instead.
Not quite as perfect in appearance, particularly in close-up photos. But my skill in painting stars, stars in circles, crosses, bars, and even numbers is good enough for my critical eye at wargaming distances and even at "pick 'em up and admire 'em" distances.
And the great advantage is that it is very little extra work if you mess one up. Just paint over the mess up, and start again.
I'm currently painting white circles with red crosses on the helmets of my medics for a H&R US infantry company. Try doing that with decals!
I'm using the ammo carriers / LMG #2s. I've clipped off their shouldered rifles. The ammo boxes they carry serve well enough as medic pouches (OK, better if they were over-the-shoulder, but hand held is not a no-no). They were all done and looking very good. White circles had been applied to 4 sides of their helmets, and to their ammo box/medic pouches, using a toothpick clipped to size (makes a useful small circle dab). Then a red razor-tip pen was used for the red crosses (hardly more than dots, actually).
But then I made the mistake of including them with the rest of the company when I sprayed them with dullcoat. D'oh! Testor's Dullcoat is a thinner for the red ink, you noob! Ran the red ink right down the sides of their helmets, across faces and necks, and onto their shoulders. :(
So they've all be re-painted (well, shoulders, packs, faces and helmets, at least), and white-dotted. And I'm ready to go with my red crosses again…
-Mark
(aka: Mk 1)