Hi Gary,
Yes 'Aviattic' decals are the way to go.
They come as either 'clear' (transparent) or 'white' (solid backing). The clear decals require a base colour applied first, whereas the white decals don't.
For those decal sheets that are not cookie cut, you'll need to cut templates out first, using the model parts as the base. The cookie cut decals are obviously already shaped and are applied in the normal way on the model parts.
Cutting templates first sounds tricky to do but is easy enough using thin paper or similar. Also the decals are robust and do have an element of stretch in them, so minor errors transferred from the cut templates can usually be sorted out during application. Although thin, any overlap on decals of the 'clear' type will show as 'double' colour, so try to avoid that.
The thing to remember is that the surface you apply the decals to must be gloss and shouldn't have any imperfections, such as dust or heavy marks, as these will show through the decals. If any particles are under the decal there's a chance that air will be trapped there causing the dreaded 'silvering, which will show through even these decals. You can either paint the base coat colour then seal it with a clear coat (I use Alclad gloss lacquer) or paint the base colour then polish it (I use 'Flory Models' polishing sticks for this)
If you apply 'clear' decals, the base coat needs to be a gloss and light colour. White is the preferred colour but you can always experiment on a piece of plastic card first to get the desired effect. Also this would allow you to try the pre-shading colour and intensity, if you want to pre-shade. It's what I do as post shading would cover the linen effect of the decal and contrast with the rest.
Once the decals have been applied and set fully, I apply a gloss coat over the areas that will have further decals applied, such as those from the kit itself.
When all decals have been applied I airbrush a sealing coat of 50-50 Alclad II Semi-Matte and Flat, which not only seal in the decals but also gives what I think is the correct amount of surface sheen.
Remember that the decals will cover any holes or location points on the model , but you can pick them out through the decal and carefully break through the dried decals, using a pin etc, to re-open them. Be careful not to pull at the decal as that can sometimes cause it to lift slightly. If that happens just apply more decal setting solution to set the decal back down, pricking through the decal first if necessary.
When that's all done you can apply your weathering techniques as desired.
