Microscale's Micro-Set is a decal "setting" solution that softens decals and improves adhesion plus lets the decal conform to the model's surface.
To apply Micro Set: Use a soft flat artist's brush and apply Micro Set where you are going to apply the decal.
See my blog page on using Micro Set.
Then slide the decal off the backing paper using a small pointed tip synthetic bristle brush to carefully position the decal in its proper position.
Blot the decal carefully with a tissue or paper toweling, being careful not to disturb its position. Allow the decal to dry overnight.
1 oz plastic bottle, by Microscale.