Here's what I do ..... others may have different approaches but this works for me.
Apply future to the area that the decals are going to be placed (or to the entore model). Because decals adhere better to gloss surfaces than flat finishes, this helps with the process. After the furture is dry, I apply a decal set solution, the decal, then a decal sol solution. After 24 to 48 hours I will apply a dullcote to bring the entire model back to a flat finish.
Happy Modelling and God Bless
Robert