What I usually do is:
- give a gloss coat to the whole model. This helps with selective washing and decal application
- dip the decals in warm water for 20-30 secs
- take the decal out f the water (still with its paper backing) and put it onto a paper tissue for a few seconds
- wet the area where the decal has to go wit a bit of plain water
- slip the decal from its support and then position it properly on the model
- with a humid cotton swab I remove the excess of water, gently rolling it over the decal. This helps to have a better decal adhesion and to remove air bubbles from underneath the decal.
- once throughly dry I apply another coat of acrylic gloss varnish to seal the decal
- once dry I coat it with matt varnish to finish the weathering.
This is with gloss undercoat and overcoat:
This is the final result: