Found this on the interwebs:
And found it very useful...
MORDHEIMER'S EDIT: Reproduced below, with correct grammar (without permission, but with credits, for preservation)Applying Decals
Step by step. By José Brito
1. Apply some Vallejo Gloss Varnish
where the decal will be placed. When well dry apply a small quantity
of Vallejo Decal Fix (73.213). For better results use the PM052 Vallejo Paint Master Brush. Vallejo Decal Fix is
a setting for decals and will increase the fixation of decal to the surface.
2. Gentle cut the decal. Use a steel ruder and a brand new blade for better results. The decal must be cut the
nearest possible to the subject.
3. Dip the decal in water for a few seconds.
4. Place the decal in the disired spot and remove the water excess. When well dry apply a small quantity of
Vallejo Decal Medium. For better results use the PM052 Vallejo Paint Master Brush. Vallejo Decal Medium is
a decal softener and some care is needed after the application over the decal. For a few hours don’t touch
the decal. Vallejo Decal Medium will do the rest! A few hours later, the decal will look just like painted. All the
recessed or raised details will be visible under the decal.
5. Once the Vallejo Decal Medium well dry, apply a small quantity of Vallejo Model Air Matt Varnish. This will
seal and protect the decal. For better results use the PM052 Vallejo Paint Master brush.