The question "how do I apply decals to the inside of a body" comes up frequently.
It used to be easy - just give the link to Tom Andersen's website which contained detailed instructions. But that website is no longer, so instead of having to re-type this every week, here is a tutorial.
There are other methods that I'm sure work fine also, but here is mine. This is for acrylic paints only.
Materials needed:
- Glazing medium, like Deco Art Americana
- Paintbrush
- Paper towel
One new trick I learned is, decals don't need to soak in a tub of water. If they stay there too long, they float off the backing. 
Instead of soaking them, dip them for a few seconds, remove and place on a piece of paper. It will be ready when you are.
When the decal is ready to slide off the backing, first brush a light film of the glaze slightly larger than the decal where you plan to place it. Then immediately place the decal against the film and remove the backing.
Position it how you want it; you have enough time to do it but if you wait too long it will start to stick. Do as many decals at the same time as you feel comfortable with.
Do not worry if some of the liquid runs, it won't affect your paint job. You can dab at runs with a paper towel and wick it up.
Also, you can also minimize runs by changing the angle of gravity periodically before it dries.
Allow to dry fully, a couple hours or so. Blow drying will accelerate the process but be careful not to overheat anything.
The Glaze is water-soluble so the brush cleans up easily.
Apply another coat. In the words of Tom Andersen "You don't want paint seeping under them." I usually use three coats.
Once your final coat is fully dry, you can begin painting. Make your first coat extremely light to seal the edges well.
