Microsoft and Microsol work well in getting decals to adhere and conform, in addition:
General theory is to trim the individual decal to the design (eliminating the spare carrier film), apply to a gloss surface and seal with whatever clear finish you are using for the model. It's safest to test both the pre decal gloss coat and the final seal with a spare decal . . . almost everyone suffers a bad reaction at some time.
Ensure that the gloss coat is dry before applying the decal and that the decal is dry before the final seal.
Painted insignia etc on real vehicles, sets and studio models were often chipped, weathered or multi shaded and if if your subject has such features then reflecting them on your model by treating/attacking the decal will help integrate the decal with the surrounding surface.
I tend to use Vallejo acrylic varnishes now applied by airbrush, but previously used spray cans without problems. Others may be braver than me but I try to stick to one manufacturer's products to avoid compatibility problems.
Masking and stencils are options in some cases.